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

[Joe Nash via llvm-commits]

Sisyph wrote:

Forming more VOPD is a reason to land this change.
It seems like some paths forward for this change could be to:

- Do more code archaeology to determine the reason the imp defs were added, and determine if that logic is still true (This may be more effort than it is worth imo)
- Do more pre-commit testing (expensive checks on cts, test on gfx908)
- Land it and wait for post-commit testing


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