[clang] [HLSL][Matrix] Make matrix truncation respect default matrix memory layout (PR #184280)

[Farzon Lotfi via cfe-commits]

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@@ -3118,14 +3118,15 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
       SmallVector<int> Mask;
       unsigned NumCols = MatTy->getNumColumns();
       unsigned NumRows = MatTy->getNumRows();
-      unsigned ColOffset = NumCols;
-      if (auto *SrcMatTy = E->getType()->getAs<ConstantMatrixType>())
-        ColOffset = SrcMatTy->getNumColumns();
-      for (unsigned R = 0; R < NumRows; R++) {
-        for (unsigned C = 0; C < NumCols; C++) {
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farzonl wrote:

Why did we need to flatten this?  Is Mask Order important I thought the only thing that matters is which indices we put into the mask?

https://github.com/llvm/llvm-project/pull/184280