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

[Farzon Lotfi via cfe-commits]

farzonl wrote:

I think we should keep case 5 in the hlsl header. you should be able to use the mad intrinsic to get what you want. if you really want to convert mad to dot product for spirv we should do that in the instcombine pass in the spirv backend in a different pr.

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