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

[Stefan Pintilie via llvm-commits]

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

> Can we pre-commit this test case? Without this PR, this case should fail verification?

Well, the issue is that without the fix in `RegisterCoalescer.cpp` the test will fail. However, without this entire patch this test will still pass. We need the fix in the register classes for this test to start failing. 

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