[clang] [llvm] [HLSL][DirectX] Implement HLSL `mul` function and DXIL lowering of `llvm.matrix.multiply` (PR #184882)

[Steven Perron via cfe-commits]

https://github.com/s-perron commented:

LGTM. I just have some suggestions on how to improve the comments. I think they could be confusing because the assume knowledge about something from a different part of the code with no way to find it.

https://github.com/llvm/llvm-project/pull/184882