[llvm] [PPC] Add DwarfRegAlias for VSRPair (PR #95837)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 approved this pull request.

LGTM.

I created https://github.com/llvm/llvm-project/issues/96129 for the follow ups related to refactor of `getDwarfRegNum()`

https://github.com/llvm/llvm-project/pull/95837