[LV] Use SCEV to check if the trip count <= VF * UF.

Just comparing constant trip counts causes LV to miss cases where the
vector loop body only executes once.

The motivation for this is to remove the need for unrolling to remove
vector loop back-edges, if the body only executes once in more cases.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D133017

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -482,10 +482,11 @@ void VPlanTransforms::optimizeForVFAndUF(VPlan &Plan, ElementCount BestVF,
   Type *IdxTy =
       Plan.getCanonicalIV()->getStartValue()->getLiveInIRValue()->getType();
   const SCEV *TripCount = createTripCountSCEV(IdxTy, PSE);
-  auto *C = dyn_cast<SCEVConstant>(TripCount);
   ScalarEvolution &SE = *PSE.getSE();
-  if (!C || TripCount->isZero() ||
-      C->getAPInt().getZExtValue() > BestVF.getKnownMinValue() * BestUF)
+  const SCEV *C =
+      SE.getConstant(TripCount->getType(), BestVF.getKnownMinValue() * BestUF);
+  if (TripCount->isZero() ||
+      !SE.isKnownPredicate(CmpInst::ICMP_ULE, TripCount, C))
     return;
 
   LLVMContext &Ctx = SE.getContext();

llvm/test/Transforms/LoopVectorize/vector-loop-backedge-elimination.ll

@@ -11,11 +11,9 @@ define void @test_tc_less_than_16(ptr %A, i64 %N) {
 ; VF8UF1:       [[CMP:%.+]] = icmp eq i64 %index.next, %n.vec
 ; VF8UF1-NEXT:  br i1 [[CMP]], label %middle.block, label %vector.body
 ;
-; VF8UF2:       [[CMP:%.+]] = icmp eq i64 %index.next, %n.vec
-; VF8UF2-NEXT:  br i1 [[CMP]], label %middle.block, label %vector.body
+; VF8UF2:       br i1 true, label %middle.block, label %vector.body
 ;
-; VF16UF1:       [[CMP:%.+]] = icmp eq i64 %index.next, %n.vec
-; VF16UF1-NEXT:  br i1 [[CMP]], label %middle.block, label %vector.body
+; VF16UF1:      br i1 true, label %middle.block, label %vector.body
 ;
 entry:
   %and = and i64 %N, 15