[llvm] [WIP][PowerPC] Add phony subregisters to cover the high half of the VSX registers. (PR #94628)

[Stefan Pintilie via llvm-commits]

https://github.com/stefanp-ibm commented:

I think this patch is ready for another review. I will also remove the [WIP] from the title as it is no longer a work in progress. 

https://github.com/llvm/llvm-project/pull/94628