[llvm] [NFC][AMDGPU] Remove an obsolete debug assertion trigger (PR #127357)

[Matt Arsenault via llvm-commits]

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@@ -512,10 +512,6 @@ bool PhiLoweringHelper::lowerPhis() {
              DT->getNode(RHS.Block)->getDFSNumIn();
     });
 
-#ifndef NDEBUG
-    PhiRegisters.insert(DstReg);
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arsenm wrote:

There is still a count call, and the member exists. I'm not sure I follow what removing this fixes, but this should remove all the parts 

https://github.com/llvm/llvm-project/pull/127357