[llvm] [IVDesc] Check loop-preheader for loop-legality when pass-remarks enabled (PR #166310)

[Florian Hahn via llvm-commits]

================
@@ -282,7 +282,9 @@ bool RecurrenceDescriptor::AddReductionVar(
 
   // Obtain the reduction start value from the value that comes from the loop
   // preheader.
-  Value *RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
+  Value *RdxStart;
+  if (TheLoop->getLoopPreheader())
+    RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
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fhahn wrote:

```suggestion
  if (TheLoop->getLoopPreheader())
    return false;
    
  Value *RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
```

https://github.com/llvm/llvm-project/pull/166310