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

[Kai Luo via llvm-commits]

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@@ -750,25 +750,21 @@ entry:
 define <2 x double> @testDoubleImm1(<2 x double> %a, double %b) {
 ; CHECK-64-LABEL: testDoubleImm1:
 ; CHECK-64:       # %bb.0: # %entry
-; CHECK-64-NEXT:    # kill: def $f1 killed $f1 def $vsl1
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bzEq wrote:

> Kill can tell the later pass that the f1/vsl1 is killed before instruction COPY

I think f1/vsl1 is killed at the COPY, not before, so that this identity COPY is replaced by a `KILL` instruction. I'm wondering how we get rid of the identity COPY after adding phony registers.

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