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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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.

Rematerialize of undef subreg defs is handled, which the base isTriviallyReMaterializable already filters out. That case is also looking through virtual register splits, which won't introduce random mismatches 

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