[LoopVectorize] In LoopVectorize start using getSymbolicMaxBackedgeTakenCount

LoopVectorizationLegality currently only treats a loop as legal
to vectorise if PredicatedScalarEvolution::getBackedgeTakenCount
returns a valid SCEV, or more precisely that the loop must have
an exact backedge taken count. Therefore, in LoopVectorize.cpp
we can safely replace all calls to getBackedgeTakenCount with
calls to getSymbolicMaxBackedgeTakenCount, since the result is
the same.

This also helps prepare the loop vectoriser for PR #88385.

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -907,7 +907,7 @@ Value *getRuntimeVF(IRBuilderBase &B, Type *Ty, ElementCount VF) {
 
 const SCEV *createTripCountSCEV(Type *IdxTy, PredicatedScalarEvolution &PSE,
                                 Loop *OrigLoop) {
-  const SCEV *BackedgeTakenCount = PSE.getBackedgeTakenCount();
+  const SCEV *BackedgeTakenCount = PSE.getSymbolicMaxBackedgeTakenCount();
   assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCount) && "Invalid loop count");
 
   ScalarEvolution &SE = *PSE.getSE();
@@ -4090,7 +4090,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     unsigned MaxVFtimesIC =
         UserIC ? *MaxPowerOf2RuntimeVF * UserIC : *MaxPowerOf2RuntimeVF;
     ScalarEvolution *SE = PSE.getSE();
-    const SCEV *BackedgeTakenCount = PSE.getBackedgeTakenCount();
+    const SCEV *BackedgeTakenCount = PSE.getSymbolicMaxBackedgeTakenCount();
     const SCEV *ExitCount = SE->getAddExpr(
         BackedgeTakenCount, SE->getOne(BackedgeTakenCount->getType()));
     const SCEV *Rem = SE->getURemExpr(
@@ -9584,7 +9584,7 @@ static bool processLoopInVPlanNativePath(
     ProfileSummaryInfo *PSI, LoopVectorizeHints &Hints,
     LoopVectorizationRequirements &Requirements) {
 
-  if (isa<SCEVCouldNotCompute>(PSE.getBackedgeTakenCount())) {
+  if (isa<SCEVCouldNotCompute>(PSE.getSymbolicMaxBackedgeTakenCount())) {
     LLVM_DEBUG(dbgs() << "LV: cannot compute the outer-loop trip count\n");
     return false;
   }