[SCEV] Avoid range intersection idiom in getRangeForUnkownRecurrence [NFC]

Addresses a review comment from D101181

lib/Analysis/ScalarEvolution.cpp

@@ -5654,7 +5654,7 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
   const DataLayout &DL = getDataLayout();
 
   unsigned BitWidth = getTypeSizeInBits(U->getType());
-  ConstantRange CR(BitWidth, /*isFullSet=*/true);
+  const ConstantRange FullSet(BitWidth, /*isFullSet=*/true);
 
   // Match a simple recurrence of the form: <start, ShiftOp, Step>, and then
   // use information about the trip count to improve our available range.  Note
@@ -5666,19 +5666,19 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
   // below intentionally handles the case where step is not loop invariant.
   auto *P = dyn_cast<PHINode>(U->getValue());
   if (!P)
-    return CR;
+    return FullSet;
 
   // Make sure that no Phi input comes from an unreachable block. Otherwise,
   // even the values that are not available in these blocks may come from them,
   // and this leads to false-positive recurrence test.
   for (auto *Pred : predecessors(P->getParent()))
     if (!DT.isReachableFromEntry(Pred))
-      return CR;
+      return FullSet;
 
   BinaryOperator *BO;
   Value *Start, *Step;
   if (!matchSimpleRecurrence(P, BO, Start, Step))
-    return CR;
+    return FullSet;
 
   // If we found a recurrence in reachable code, we must be in a loop. Note
   // that BO might be in some subloop of L, and that's completely okay.
@@ -5690,12 +5690,12 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
     // with malformed loop information during the midst of the transform.
     // There doesn't appear to be an obvious fix, so for the moment bailout
     // until the caller issue can be fixed.  PR49566 tracks the bug.
-    return CR;
+    return FullSet;
 
   // TODO: Extend to other opcodes such as mul, and div
   switch (BO->getOpcode()) {
   default:
-    return CR;
+    return FullSet;
   case Instruction::AShr:
   case Instruction::LShr:
   case Instruction::Shl:
@@ -5704,11 +5704,11 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
 
   if (BO->getOperand(0) != P)
     // TODO: Handle the power function forms some day.
-    return CR;
+    return FullSet;
 
   unsigned TC = getSmallConstantMaxTripCount(L);
   if (!TC || TC >= BitWidth)
-    return CR;
+    return FullSet;
 
   auto KnownStart = computeKnownBits(Start, DL, 0, &AC, nullptr, &DT);
   auto KnownStep = computeKnownBits(Step, DL, 0, &AC, nullptr, &DT);
@@ -5721,7 +5721,7 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
   bool Overflow = false;
   auto TotalShift = MaxShiftAmt.umul_ov(TCAP, Overflow);
   if (Overflow)
-    return CR;
+    return FullSet;
 
   switch (BO->getOpcode()) {
   default:
@@ -5734,17 +5734,14 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
     // Thus, the end value is closer to zero than the start.
     auto KnownEnd = KnownBits::ashr(KnownStart,
                                     KnownBits::makeConstant(TotalShift));
-    if (KnownStart.isNonNegative()) {
+    if (KnownStart.isNonNegative())
       // Analogous to lshr (simply not yet canonicalized)
-      auto R = ConstantRange::getNonEmpty(KnownEnd.getMinValue(),
-                                          KnownStart.getMaxValue() + 1);
-      CR = CR.intersectWith(R);
-    } else if (KnownStart.isNegative()) {
+      return ConstantRange::getNonEmpty(KnownEnd.getMinValue(),
+                                        KnownStart.getMaxValue() + 1);
+    if (KnownStart.isNegative())
       // End >=u Start && End <=s Start
-      auto R = ConstantRange::getNonEmpty(KnownStart.getMinValue(),
-                                          KnownEnd.getMaxValue() + 1);
-      CR = CR.intersectWith(R);
-    }
+      return ConstantRange::getNonEmpty(KnownStart.getMinValue(),
+                                        KnownEnd.getMaxValue() + 1);
     break;
   }
   case Instruction::LShr: {
@@ -5755,26 +5752,22 @@ getRangeForUnknownRecurrence(const SCEVUnknown *U) {
     // Thus, the low end of the unsigned range is the last value produced.
     auto KnownEnd = KnownBits::lshr(KnownStart,
                                     KnownBits::makeConstant(TotalShift));
-    auto R = ConstantRange::getNonEmpty(KnownEnd.getMinValue(),
-                                        KnownStart.getMaxValue() + 1);
-    CR = CR.intersectWith(R);
-    break;
+    return ConstantRange::getNonEmpty(KnownEnd.getMinValue(),
+                                      KnownStart.getMaxValue() + 1);
   }
   case Instruction::Shl: {
     // Iff no bits are shifted out, value increases on every shift.
     auto KnownEnd = KnownBits::shl(KnownStart,
                                    KnownBits::makeConstant(TotalShift));
     if (TotalShift.ult(KnownStart.countMinLeadingZeros()))
-      CR = CR.intersectWith(ConstantRange(KnownStart.getMinValue(),
-                                          KnownEnd.getMaxValue() + 1));
+      return ConstantRange(KnownStart.getMinValue(),
+                           KnownEnd.getMaxValue() + 1);
     break;
   }
   };
-  return CR;
+  return FullSet;
 }
 
-
-
 /// Determine the range for a particular SCEV.  If SignHint is
 /// HINT_RANGE_UNSIGNED (resp. HINT_RANGE_SIGNED) then getRange prefers ranges
 /// with a "cleaner" unsigned (resp. signed) representation.