[llvm] [AMDGPU] Stop adding implicit def of superreg in copyPhysReg (PR #125255)

[Jay Foad via llvm-commits]

jayfoad wrote:

What was the point of these implicit _defs_ in the first place? I've tried going back to some related older commits like e56e9022bc54c575ad8ecc3934e9cf84b8542e62 but I still can't reproduce any liveness verification problems if I remove the implicit defs.

https://github.com/llvm/llvm-project/pull/125255