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

[Lei Huang via llvm-commits]

https://github.com/lei137 commented:

I looked through all the tests that this patch updated and it seem to be doing what was expected so that part LGTM.

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