[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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@@ -3672,6 +3672,14 @@ bool RegisterCoalescer::joinVirtRegs(CoalescerPair &CP) {
     // having stale segments.
     LHSVals.pruneMainSegments(LHS, ShrinkMainRange);
 
+    LHSVals.pruneSubRegValues(LHS, ShrinkMask);
+    RHSVals.pruneSubRegValues(LHS, ShrinkMask);
+  } else if (TrackSubRegLiveness && !CP.getDstIdx() && CP.getSrcIdx()) {
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stefanp-ibm wrote:

> Is it possible to split this change and test into its own PR

The issue with splitting this test and change away is that the test passes anyway unless it is accompanied by the subregsiter changes in this PR.  So, unfortunately, if I split out the test and add it to top of trunk main the test will PASS even without the changes to `RegisterCoalescer.cpp`. The error is exposed by the subregister changes that we made in `PPCRegisterInfo.td`. 

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