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

[Kai Luo via llvm-commits]

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+# RUN: llc -mtriple powerpc64le-unknown-linux-gnu -mcpu=pwr8 -x mir < %s \
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bzEq wrote:

Ok, I see. The `HasDisjunctSubRegs` bit is false without this PR, so that `MRI->shouldTrackSubRegLiveness(...)` returns false.

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