[llvm] [DirectX] Start the creation of a DXIL Instruction legalizer (PR #131221)

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

A fairly minor style comment and a couple places we could probably add asserts that are probably worth doing if you're planning on more updates in the area.

https://github.com/llvm/llvm-project/pull/131221