[clang] Clang][HLSL] Emit constant matrix values in column-major order (PR #227356)

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@llvm/pr-subscribers-hlsl

Author: Farzon Lotfi (farzonl)

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<summary>Changes</summary>

Emit matrix APValues in the canonical column-major register layout, independent of the selected matrix memory layout.

When a constant is used as a row-major memory initializer, reorder its elements at the register-to-memory boundary. This preserves the physical layout of global and static row-major matrices while ensuring constexpr matrix expressions produce canonical register values.

Assisted by Cop-pilot GPT 5.6-Sol

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Full diff: https://github.com/llvm/llvm-project/pull/227356.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGExprConstant.cpp (+12-3) 
- (modified) clang/test/CodeGenHLSL/BasicFeatures/MatrixInitializerListOrder.hlsl (+11) 


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https://github.com/llvm/llvm-project/pull/227356