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

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: 53c37f300dd1b450671f2aee4cc649c380adb5ad

https://github.com/llvm/llvm-project/actions/runs/10199739617

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

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