[llvm] [NFC][CodeGen] Add helper function to check SubReg validity (PR #181489)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

The two arg form of `isSubRegValidForRegClass` seems pretty uncontroverial. Unless you want to bikeshed the name, e.g. how about `regClassSupportsSubReg`?

I'm less keen on the three arg overload just because it's more complex, seems less generally useful, and only has a single use.

https://github.com/llvm/llvm-project/pull/181489