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

[Shengchen Kan via llvm-commits]

KanRobert wrote:

> > @KanRobert with the changed tests, there are visible codegen changes. There are no machine verifier failures, as there should be if this were truly invalid. Can you take on using RegClassByHwMode for this? That would get you free verifier errors and then we can drop the virtual on getRegClass
> 
> You will get failures when compiling to binary, e.g., `-filetype=obj`

@phoebewang Should we add `-filetype=obj` to test to check no error too? This way, developers don’t need to be aware of the limitations of x86 encoding when performing refactoring.

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