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

[Kai Luo via llvm-commits]

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@@ -168,15 +175,21 @@ foreach Index = 0-31 in {
                  DwarfRegNum<[!add(Index, 77), !add(Index, 77)]>;
 }
 
+let isArtificial = 1 in {
+  foreach Index = 0-31 in {
+    def VFH#Index : VF<-1, "">;
+  }
+}
+
 // Vector registers
 foreach Index = 0-31 in {
-  def V#Index : VR<!cast<VF>("VF"#Index), "v"#Index>,
-                DwarfRegNum<[!add(Index, 77), !add(Index, 77)]>;
+  def V#Index : VR<!cast<VF>("VF"#Index), !cast<VF>("VFH"#Index), "v"#Index>,
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bzEq wrote:

Maybe we also need to update VRRC to be `CoveredBySubRegs`.

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