[llvm] [AMDGPU] Add IR LiveReg type-based optimization (PR #66838)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

Introduce BBUseValMap to correctly track per BB the conversions back to the original type for users + handle zeroinitializer incoming values & return on unhandled incoming values.

Latest version passes PSDB

https://github.com/llvm/llvm-project/pull/66838