[clang] [Clang] Remove 3-element vector load and store special handling (PR #104661)

[Matt Arsenault via cfe-commits]

================
@@ -2145,21 +2144,21 @@ void CodeGenFunction::EmitStoreOfScalar(llvm::Value *Value, Address Addr,
       Addr = Addr.withPointer(Builder.CreateThreadLocalAddress(GV),
                               NotKnownNonNull);
 
+  // Handles vectors of sizes that are likely to be expanded to a larger size
+  // to optimize performance.
   llvm::Type *SrcTy = Value->getType();
   if (const auto *ClangVecTy = Ty->getAs<VectorType>()) {
-    auto *VecTy = dyn_cast<llvm::FixedVectorType>(SrcTy);
-    if (!CGM.getCodeGenOpts().PreserveVec3Type) {
-      // Handle vec3 special.
-      if (VecTy && !ClangVecTy->isExtVectorBoolType() &&
-          cast<llvm::FixedVectorType>(VecTy)->getNumElements() == 3) {
-        // Our source is a vec3, do a shuffle vector to make it a vec4.
-        Value = Builder.CreateShuffleVector(Value, ArrayRef<int>{0, 1, 2, -1},
-                                            "extractVec");
-        SrcTy = llvm::FixedVectorType::get(VecTy->getElementType(), 4);
+    if (auto *VecTy = dyn_cast<llvm::FixedVectorType>(SrcTy)) {
+      auto *NewVecTy =
+          CGM.getABIInfo().getOptimalVectorMemoryType(VecTy, getLangOpts());
+      if (!ClangVecTy->isExtVectorBoolType() && VecTy != NewVecTy) {
+        SmallVector<int, 4> Mask(NewVecTy->getNumElements(), -1);
----------------
arsenm wrote:

```suggestion
        SmallVector<int, 16> Mask(NewVecTy->getNumElements(), -1);
```

https://github.com/llvm/llvm-project/pull/104661