[clang] [HLSL][Matrix] Implement dynamic single subscript swizzle (PR #173201)

[Helena Kotas via cfe-commits]

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@@ -2471,17 +2471,27 @@ RValue CodeGenFunction::EmitLoadOfLValue(LValue LV, SourceLocation Loc) {
 
     unsigned NumRows = MT->getNumRows();
     unsigned NumCols = MT->getNumColumns();
-
+    unsigned NumLanes = NumCols;
     llvm::Value *MatrixVec = EmitLoadOfScalar(LV, Loc);
     llvm::Value *Row = LV.getMatrixRowIdx();
     llvm::Type *ElemTy = ConvertType(MT->getElementType());
     llvm::Type *RowTy = llvm::FixedVectorType::get(ElemTy, MT->getNumColumns());
     llvm::Value *Result = llvm::PoisonValue::get(RowTy); // <NumCols x T>
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hekota wrote:

```suggestion
    llvm::Value *Result = nullptr; // will be <NumLanes x T> once NumLanes is known
```
The `Result` value needs to be created later since its vector size depends on the swizzle element count.

https://github.com/llvm/llvm-project/pull/173201