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

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

This does not seem safe unless you audit all uses of `isFullCopyInstr`. The first one I looked at is in `VirtRegAuxInfo::isRematerializable` which seems like it really does want whole-register-to-whole-register copies.

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