[SCEV] Generalize getSmallConstantTripCount(L) for multiple exit loops

This came up in review for another patch, see https://reviews.llvm.org/D102982#2782407 for full context.

I've reviewed the callers to make sure they can handle multiple exit loops w/non-zero returns.  There's two cases in target cost models where results might change (Hexagon and PowerPC), but the results looked legal and reasonable.  If a target maintainer wishes to back out the effect of the costing change, they should explicitly check for multiple exit loops and handle them as desired.

Differential Revision: https://reviews.llvm.org/D103182

include/llvm/Analysis/ScalarEvolution.h

@@ -712,17 +712,18 @@ public:
   /// 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
+  /// Returns the exact trip count of the loop if we can compute it, and
-  /// loop and we can compute a small maximum for that loop.
+  /// the result is a small constant.  '0' is used to represent an unknown
-  ///
+  /// or non-constant trip count.  Note that a trip count is simply one more
-  /// Implemented in terms of the \c getSmallConstantTripCount overload with
+  /// than the backedge taken count for the loop.
-  /// the single exiting block passed to it. See that routine for details.
   unsigned getSmallConstantTripCount(const Loop *L);
 
-  /// Returns the maximum trip count of this loop as a normal unsigned
+  /// Return the exact trip count for this loop if we exit through ExitingBlock.
-  /// value. Returns 0 if the trip count is unknown or not constant. This
+  /// '0' is used to represent an unknown or non-constant trip count.  Note
-  /// "trip count" assumes that control exits via ExitingBlock. More
+  /// that a trip count is simply one more than the backedge taken count for
-  /// precisely, it is the number of times that control may reach ExitingBlock
+  /// the same exit.
+  /// This "trip count" assumes that control exits via ExitingBlock. More
+  /// precisely, it is the number of times that control will reach ExitingBlock
   /// before taking the branch. For loops with multiple exits, it may not be
   /// the number times that the loop header executes if the loop exits
   /// prematurely via another branch.

lib/Analysis/ScalarEvolution.cpp

@@ -6948,11 +6948,8 @@ static unsigned getConstantTripCount(const SCEVConstant *ExitCount) {
 }
 
 unsigned ScalarEvolution::getSmallConstantTripCount(const Loop *L) {
-  if (BasicBlock *ExitingBB = L->getExitingBlock())
+  auto *ExitCount = dyn_cast<SCEVConstant>(getBackedgeTakenCount(L, Exact));
-    return getSmallConstantTripCount(L, ExitingBB);
+  return getConstantTripCount(ExitCount);
-
-  // No trip count information for multiple exits.
-  return 0;
 }
 
 unsigned