[SCEV] Add a utility for converting from "exit count" to "trip count"

(Mostly as a logical place to put a comment since this is a reoccuring confusion.)

include/llvm/Analysis/ScalarEvolution.h

@@ -705,6 +705,13 @@ public:
   bool isLoopBackedgeGuardedByCond(const Loop *L, ICmpInst::Predicate Pred,
                                    const SCEV *LHS, const SCEV *RHS);
 
+  /// Convert from an "exit count" (i.e. "backedge taken count") to a "trip
+  /// count".  A "trip count" is the number of times the header of the loop
+  /// will execute if an exit is taken after the specified number of backedges
+  /// have been taken.  (e.g. TripCount = ExitCount + 1)  A zero result
+  /// must be interpreted as a loop having an unknown trip count.
+  const SCEV *getTripCountFromExitCount(const SCEV *ExitCount);
+
   /// Returns the maximum trip count of the loop if it is a single-exit
   /// loop and we can compute a small maximum for that loop.
   ///

lib/Analysis/LoopCacheAnalysis.cpp

@@ -109,9 +109,7 @@ static const SCEV *computeTripCount(const Loop &L, ScalarEvolution &SE) {
   if (isa<SCEVCouldNotCompute>(BackedgeTakenCount) ||
       !isa<SCEVConstant>(BackedgeTakenCount))
     return nullptr;
-
-  return SE.getAddExpr(BackedgeTakenCount,
-                       SE.getOne(BackedgeTakenCount->getType()));
+  return SE.getTripCountFromExitCount(BackedgeTakenCount);
 }
 
 //===----------------------------------------------------------------------===//

lib/Analysis/ScalarEvolution.cpp

@@ -6927,6 +6927,12 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
 //                   Iteration Count Computation Code
 //
 
+const SCEV *ScalarEvolution::getTripCountFromExitCount(const SCEV *ExitCount) {
+  // Get the trip count from the BE count by adding 1.  Overflow, results
+  // in zero which means "unknown".
+  return getAddExpr(ExitCount, getOne(ExitCount->getType()));
+}
+
 static unsigned getConstantTripCount(const SCEVConstant *ExitCount) {
   if (!ExitCount)
     return 0;
@@ -6979,8 +6985,8 @@ unsigned ScalarEvolution::getSmallConstantTripMultiple(const Loop *L,
   if (ExitCount == getCouldNotCompute())
     return 1;
 
-  // Get the trip count from the BE count by adding 1.
-  const SCEV *TCExpr = getAddExpr(ExitCount, getOne(ExitCount->getType()));
+  // Get the trip count
+  const SCEV *TCExpr = getTripCountFromExitCount(ExitCount);
 
   const SCEVConstant *TC = dyn_cast<SCEVConstant>(TCExpr);
   if (!TC)