[HLSL][DirectX] Implement HLSL `mul` function and DXIL lowering of `llvm.matrix.multiply` (#184882) Fixes #99138 - Defines a `__builtin_hlsl_mul` clang builtin in `Builtins.td`. - Links the `__builtin_hlsl_mul` clang builtin with `hlsl_alias_intrinsics.h` under the name `mul` for matrix cases - Implement scalar and vector elementwise multiplication cases of the `mul` function in `hlsl_intrinsics.h` and `hlsl_intrinsic_helpers.h` - Adds sema for `__builtin_hlsl_mul` to `CheckBuiltinFunctionCall` in `SemaHLSL.cpp` - Adds codegen for `__builtin_hlsl_mul` to `EmitHLSLBuiltinExpr` in `CGHLSLBuiltins.cpp` - Vector-vector cases lower to `dot` (except double vectors, which expands to scalar multiply-adds). - Matrix-matrix, matrix-vector, and vector-matrix multiplication lower to the `llvm.matrix.multiply` intrinsic - Adds codegen tests to `clang/test/CodeGenHLSL/builtins/mul.hlsl` - Adds sema tests to `clang/test/SemaHLSL/BuiltIns/mul-errors.hlsl` - Implements lowering of the `llvm.matrix.multiply` intrinsic to DXIL in `DXILIntrinsicExpansion.cpp` Note: Currently the SPIRV backend does not support row-major matrix memory layouts when lowering matrix multiply, and just assumes column-major layout. Therefore this PR also makes the DirectX backend only assume column-major layout. Implementing support for row-major order shall be done in a separate PR. (Tracked by https://github.com/llvm/llvm-project/issues/184906) This PR locally passes the `mul` offload tests in both DirectX 12 and Vulkan: https://github.com/llvm/offload-test-suite/pull/941 Assisted-by: claude-opus-4.6
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