[llvm] [1/3][AMDGPU][NFC] Add virtual prefix to GCN trackers (PR #203667)

[Dhruva Chakrabarti via llvm-commits]

dhruvachak wrote:

Added virtual prefix to more methods and local variables. These renames seem to make sense as the scope of physical register tracking expands in https://github.com/llvm/llvm-project/pull/184275.

https://github.com/llvm/llvm-project/pull/203667