[llvm] [SimpleLoopUnswitch] Allow trivial unswitch when exit LCSSA is a header PHI (PR #222962)

[Dark Steve via llvm-commits]

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@@ -790,6 +807,18 @@ static bool unswitchTrivialBranch(Loop &L, CondBrInst &BI, DominatorTree &DT,
     rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB,
                                               *ParentBB, *OldPH, FullUnswitch);
 
+  // OldPH branches here without entering the loop, so a header PHI is still
+  // its entry value. Read it from NewPH, the preheader created by the split.
+  if (TrivialFromHeader)
+    for (PHINode &PN : UnswitchedBB->phis())
+      for (unsigned I = 0, E = PN.getNumIncomingValues(); I != E; ++I) {
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PrasoonMishra wrote:

Done. 
(Added the braces to the `if` block as that seemed cleaner)

https://github.com/llvm/llvm-project/pull/222962