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

[Stefan Pintilie via llvm-commits]

stefanp-ibm wrote:

I've split a patch from this one:
https://github.com/llvm/llvm-project/pull/96839

It contains two tests. One is for X86 which actually fails without the fix to the register coalescer and the other is the PowerPC test that passes without the PowerPC subregister changes. 

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