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

[Shengchen Kan via llvm-commits]

KanRobert wrote:

> > It seems this PR should not only remove the overriding. If there is an alternative/better way to achieve that
> 
> The original PR already reserved these registers
> 
> > > it should probably generalize the LookupPtrRegClass mechanism.
> > 
> > 
> > , it should be included too, otherwise there is a correctness issue.
> 
> That would be necessary if the underlying class fundamentally changed (e.g. 32-bit to 64-bit), but this just swapping to a different subset, which also aligns with the reserved registers so I don't think there's anything to do here.

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. 

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