[llvm] [LoopPeel] LCSSA form is destroyed by LoopPeel, preserve it (PR #78696)

[Vedant Paranjape via llvm-commits]

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@@ -644,6 +644,13 @@ bool SimplifyIndvar::replaceIVUserWithLoopInvariant(Instruction *I) {
 
   auto *Invariant = Rewriter.expandCodeFor(S, I->getType(), IP);
 
+  if (!LI->replacementPreservesLCSSAForm(I, Invariant)) {
+    LLVM_DEBUG(dbgs() << "INDVARS: Can not replace IV user: " << *I
+                      << " with loop invariant: " << *S
+                      << " as it breaks LCSSA form " << '\n');
+    return false;
+  }
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vedantparanjape-amd wrote:

There's call to `fixupLCSSAFormFor` in SCEVExpander::expand, which in turn calls formLCSSAForInstructions. So, I am not sure why it's failing to insert a LCSSA Phi

https://github.com/llvm/llvm-project/pull/78696