[VectorCombine] scalarizeBinopOrCmp - check for out of bounds element indices

Fixes #115575

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -952,6 +952,12 @@ bool VectorCombine::scalarizeBinopOrCmp(Instruction &I) {
   if (!IsConst0 && !IsConst1 && Index0 != Index1)
     return false;
 
+  auto *VecTy0 = cast<VectorType>(Ins0->getType());
+  auto *VecTy1 = cast<VectorType>(Ins1->getType());
+  if (VecTy0->getElementCount().getKnownMinValue() <= Index0 ||
+      VecTy1->getElementCount().getKnownMinValue() <= Index1)
+    return false;
+
   // Bail for single insertion if it is a load.
   // TODO: Handle this once getVectorInstrCost can cost for load/stores.
   auto *I0 = dyn_cast_or_null<Instruction>(V0);

llvm/test/Transforms/VectorCombine/X86/pr115575.ll

@@ -4,8 +4,8 @@
 define <2 x i8> @PR115575(i8 %x) {
 ; CHECK-LABEL: define <2 x i8> @PR115575(
 ; CHECK-SAME: i8 [[X:%.*]]) {
-; CHECK-NEXT:    [[BO_SCALAR:%.*]] = sdiv i8 [[X]], poison
-; CHECK-NEXT:    [[BO:%.*]] = insertelement <2 x i8> poison, i8 [[BO_SCALAR]], i64 3
+; CHECK-NEXT:    [[INS:%.*]] = insertelement <2 x i8> poison, i8 [[X]], i32 3
+; CHECK-NEXT:    [[BO:%.*]] = sdiv <2 x i8> [[INS]], <i8 5, i8 2>
 ; CHECK-NEXT:    ret <2 x i8> [[BO]]
 ;
   %ins = insertelement <2 x i8> poison, i8 %x, i32 3