[llvm] CodeGen: Treat subreg-to-subreg copies as isFullCopyInstr (PR #120056)

[Quentin Colombet via llvm-commits]

qcolombet wrote:

I did a quick audit of all the places that `isFullCopyInstr` is used and that might be fine.

However, given this function is kind of a mirror of `MachineInstr::isFullCopy` (at least the names are close enough that if their behavior differ it may be confusing) and that one I think some users assume that the subreg is 0, I am not particularly keen on this one change.

Could you double check how `MachineInstr::isFullCopy` is used?

https://github.com/llvm/llvm-project/pull/120056