[SLP][NFC] Restructure getInsertIndex

[skc7]

Summary

Fix getInsertIndex to correctly cast to FixedVectorType.
This is required because insertElement instructions can use scalable vectors aswell.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

292–293

Could you simplify it?

const auto *VT = dyn_cast<FixedVectorType>(IE->getType());
if (!VT)
  return None;
        
const auto *CI = dyn_cast<ConstantInt>(IE->getOperand(2));
if (!CI)
  return None;

if (CI->getValue().uge(VT->getNumElements()))
  return None;

Index *= VT->getNumElements();
Index += CI->getZExtValue();
return Index;

[skc7]

changes as per review suggestion.

[ABataev]

LG

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

static bool isCommutative(Instruction *I) {
   return false;
 }
 
 /// \returns inserting index of InsertElement or InsertValue instruction,
 /// using Offset as base offset for index.
 static Optional<unsigned> getInsertIndex(const Value *InsertInst,
                                          unsigned Offset = 0) {
   int Index = Offset;
   if (const auto *IE = dyn_cast<InsertElementInst>(InsertInst)) {
-    if (const auto *CI = dyn_cast<ConstantInt>(IE->getOperand(2))) {
-      auto *VT = cast<FixedVectorType>(IE->getType());
-      if (CI->getValue().uge(VT->getNumElements()))
-        return None;
-      Index *= VT->getNumElements();
-      Index += CI->getZExtValue();
-      return Index;
-    }
-    return None;
-  }
+    const auto *VT = dyn_cast<FixedVectorType>(IE->getType());
+    if (!VT)
+      return None;
+    const auto *CI = dyn_cast<ConstantInt>(IE->getOperand(2));
+    if (!CI)
+      return None;
 
   const auto *IV = cast<InsertValueInst>(InsertInst);
   Type *CurrentType = IV->getType();
   for (unsigned I : IV->indices()) {
     if (const auto *ST = dyn_cast<StructType>(CurrentType)) {
       Index *= ST->getNumElements();
       CurrentType = ST->getElementType(I);
     } else if (const auto *AT = dyn_cast<ArrayType>(CurrentType)) {