[DXIL][TableGen] Define DXIL operation types instead of relying on LLVM Types

[bharadwajy]

Should we define an enum of our own set of types rather than using CodeGen/ValueTypes.td? Types like dx.handle and dx.ResRet.f32 and the like are going to be annoying to represent using MVTs, and there's actually a pretty limited set of legal types in DXIL operations.

Originally posted by @bogner in #99055 (review)

Acceptance Criteria

Replace all uses of llvm_*_ty in DXIL.td with types specifically defined to represent DXIL operator types.