[llvm] [ValueTracking] Fix crash in PR175590 (PR #180355)

[Matt Arsenault via llvm-commits]

================
@@ -213,6 +213,55 @@ for.body:                                         ; preds = %for.cond
   br label %for.cond
 }
 
+define fastcc i1 @ShouldMergeBlock(ptr %arena, i1 %exitcond) {
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arsenm wrote:

```suggestion
define i1 @ShouldMergeBlock(ptr %arena, i1 %exitcond) {
```

https://github.com/llvm/llvm-project/pull/180355