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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> It's different from the logic of reserved registers, which does not depend on the instruction itself. Here we have the check
> 
> ```
> X86II::canUseApxExtendedReg(MCID)
> ```
> 
> b/c even when feature APX/EGPR is on, some instructions can not use R16-R31.

Can you take care of this then? Consider this a post-commit review. That patch was not actually approved, and does not have adequate test coverage for this restriction 


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