[llvm-branch-commits] [TableGen] Support RegClassByHwMode in CompressPat (PR #171061)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/171061

This does not yet handle all cases but at least for the simple
cases such as:
```
def : CompressPat<(PTR_MOV PtrRC:$dst, PtrRC:$src),
                  (PTR_MOV_SMALL PtrRC:$dst, PtrRC:$src)>;
```
tablegen generates sensible code instead of emitting confusing errors.