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

[Deric C. via llvm-commits]

https://github.com/Icohedron approved this pull request.

Looks good aside from the nits I had on the code comments.
I also rebased my int64ops branch to this PR and found that it does properly fix the dxil validation issues.

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