[clang] [Matrix][HLSL] Implement Matrix single constant index swizzle (PR #173044)

[Justin Bogner via cfe-commits]

================
@@ -5239,8 +5240,28 @@ EmitExtVectorElementExpr(const ExtVectorElementExpr *E) {
     return LValue::MakeExtVectorElt(Base.getAddress(), CV, type,
                                     Base.getBaseInfo(), TBAAAccessInfo());
   }
-  if (Base.isMatrixRow())
+  if (Base.isMatrixRow()) {
+    if (auto *RowIdx =
+            llvm::dyn_cast<llvm::ConstantInt>(Base.getMatrixRowIdx())) {
+      llvm::SmallVector<llvm::Constant *, 8> MatIndices;
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bogner wrote:

Probably better to use the heuristically sized SmallVector here unless there's a particular reason that 8 is a better choice.
```suggestion
      llvm::SmallVector<llvm::Constant *> MatIndices;
```

https://github.com/llvm/llvm-project/pull/173044