[VectorCombine] Pull out isa<VectorType> check.

Noticed while investigating a crash in #119559 - we don't account for I being replaced and its Type being reallocated. So hoist the checks to the start of the loop.

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -2873,6 +2873,7 @@ bool VectorCombine::run() {
   bool MadeChange = false;
   auto FoldInst = [this, &MadeChange](Instruction &I) {
     Builder.SetInsertPoint(&I);
+    bool IsVectorType = isa<VectorType>(I.getType());
     bool IsFixedVectorType = isa<FixedVectorType>(I.getType());
     auto Opcode = I.getOpcode();
 
@@ -2895,7 +2896,7 @@ bool VectorCombine::run() {
 
     // This transform works with scalable and fixed vectors
     // TODO: Identify and allow other scalable transforms
-    if (isa<VectorType>(I.getType())) {
+    if (IsVectorType) {
       MadeChange |= scalarizeBinopOrCmp(I);
       MadeChange |= scalarizeLoadExtract(I);
       MadeChange |= scalarizeVPIntrinsic(I);