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

[Matt Arsenault via llvm-commits]

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+# RUN: llc -mcpu=pwr10 -O3 -ppc-track-subreg-liveness -verify-machineinstrs \
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arsenm wrote:

Use -verify-regalloc instead of -verify-machineinstrs. Also shouldn't need -O3 

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