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

[Florian Hahn via llvm-commits]

================
@@ -282,8 +282,11 @@ 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())
+    return false;
 
+  RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
----------------
fhahn wrote:

```suggestion
  if (!TheLoop->getLoopPreheader())
    return false;

  Value * RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
```

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