[llvm] [VectorCombine] Expand `vector_insert` into shufflevector for earlier cost optimizations (#145512) (PR #146479)

[Simon Pilgrim via llvm-commits]

================
@@ -804,6 +805,64 @@ bool VectorCombine::foldInsExtBinop(Instruction &I) {
   return true;
 }
 
+/// Try to fold vector_insert intrinsics into shufflevector instructions.
+bool VectorCombine::foldVectorInsertToShuffle(Instruction &I) {
+  auto *II = dyn_cast<IntrinsicInst>(&I);
+  // This optimization only applies to vector_insert intrinsics.
+  if (!II || II->getIntrinsicID() != Intrinsic::vector_insert)
+    return false;
+
+  Value *Vec = II->getArgOperand(0);
+  Value *SubVec = II->getArgOperand(1);
+  Value *Idx = II->getArgOperand(2);
+
+  // Caller guarantees DstTy is a fixed vector.
+  auto *DstTy = cast<FixedVectorType>(II->getType());
+  auto *VecTy = dyn_cast<FixedVectorType>(Vec->getType());
+  auto *SubVecTy = dyn_cast<FixedVectorType>(SubVec->getType());
+
+  // Only canonicalize if Vec and SubVec are both fixed vectors.
+  if (!VecTy || !SubVecTy)
+    return false;
+
+  unsigned DstNumElts = DstTy->getNumElements();
+  unsigned VecNumElts = VecTy->getNumElements();
+  unsigned SubVecNumElts = SubVecTy->getNumElements();
+  auto *SubVecPtr = dyn_cast<ConstantInt>(Idx);
+  if (!SubVecPtr)
+    return false;
+
+  unsigned SubVecIdx = SubVecPtr->getZExtValue();
+
+  // Ensure insertion of SubVec doesn't exceed Dst bounds.
+  if (SubVecIdx % SubVecNumElts != 0 || SubVecIdx + SubVecNumElts > DstNumElts)
----------------
RKSimon wrote:

avoid Wparenthesis warnings:
```
if ((SubVecIdx % SubVecNumElts) != 0 ||
    (SubVecIdx + SubVecNumElts) > DstNumElts)
```

https://github.com/llvm/llvm-project/pull/146479