[AMDGPU] Drop const for value that is copied (NFC).

This fixes

    warning: loop variable 'Def' of type 'const llvm::Register' creates a copy from type 'const llvm::Register' [-Wrange-loop-analysis]

llvm::Register just contains a single unsigned and should be copied.

Reviewers: rampitec

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D77011