[llvm] [LoopFusion] Fix sink instructions (PR #147501)

[Michael Kruse via llvm-commits]

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@@ -1176,6 +1176,27 @@ struct LoopFuser {
     return true;
   }
 
+  // This function fixes sunk PHI nodes after fusion.
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Meinersbur wrote:

```suggestion
  /// This function fixes sunk PHI nodes after fusion.
```
Could you explain a bit more? E.g.:
> Update the the PHI nodes that previously had incoming blocks from FC0 to the equivalent block fused in the fused loop (which is FC1 being reused, therefore the equivalent is not necessary for FC1).

https://github.com/llvm/llvm-project/pull/147501