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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I've reopened for the reland, the latest doesn't have the sanitizer issues flagged in #97138.
> 
> Since visitLoadInst deletes dead loads, optimizeLiveType may try to analyze freed memory. Simply deferring the deletion until post-loop doesn't work as the loads marked for removal may be part of a subsequently coerced chain. In order to integrate the loops, these interactions will need to be handled, which is left as a TODO.

This pass, like most combiners, should work bottom up not top down. Would that avoid the issue? 

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