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

[Quentin Colombet 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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qcolombet wrote:

If CP has a src idx, I would expect the source value to have subranges when `TrackSubRegLiveness` is on.
Bottom line, I don't see why we need this case and thus echoing what @arsenm said wrt adding a test.

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