[llvm] AMDGPU: Don't COPY constants from VGPR to SGPR (PR #66882)

[Joe Nash via llvm-commits]

https://github.com/Sisyph approved this pull request.

LGTM except for the commit title nit. The title seems accurate, but slightly misleading because we  duplicate the constants into SGPRs instead of COPY them.

https://github.com/llvm/llvm-project/pull/66882