[llvm] X86: Stop overriding getRegClass (PR #155128)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > @KanRobert @arsenm Whats this plan for this PR please?
> 
> I suppose @arsenm would re-implement this after rebasing.

@KanRobert I can take care of reposting this in its current delete-as-is form. You previously claimed this will break something, but the current test coverage is inadequate and this approach insufficient. I have no intention of taking on that work of fixing the test coverage or switching to use RegClassByHwMode, I'm asking you as the original submitter of the implementation to take care of that 

https://github.com/llvm/llvm-project/pull/155128