[llvm] [AMDGPU] Reland: Do not use original PHIs in coercion chains (PR #98985)

[Matt Arsenault via llvm-commits]

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@@ -365,13 +365,33 @@ bool LiveRegOptimizer::optimizeLiveType(
       else
         MissingIncVal = true;
     }
-    Instruction *DeadInst = Phi;
     if (MissingIncVal) {
-      DeadInst = cast<Instruction>(ValMap[Phi]);
-      // Do not use the dead phi
-      ValMap[Phi] = Phi;
+      Value *DeadVal = ValMap[Phi];
+      // The coercion chain of the PHI is broken. Delete the Phi
+      // from the ValMap and any connected / user Phis.
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arsenm wrote:

But if you do RAUW, the weak handles handle the case where the value is ultimately deleted 

https://github.com/llvm/llvm-project/pull/98985