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

[Stefan Pintilie via llvm-commits]

stefanp-ibm wrote:

Note that this change goes on top of:
https://github.com/llvm/llvm-project/pull/96839

However, I can't seem to get git to show only the diff after that change. So, not compared to main but compared to [stefanp/RegCoalesceFix]. Therefore, this is the link to see that smaller diff:
https://github.com/llvm/llvm-project/pull/94628/files/6c707edffd1e368461cf50cc0068f2c29ece56d8..df2892d03bb5f7cde6f84bacf8ecc267c5dc9212

Once that patch goes in then I can rebase properly and compare against main. 

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