[llvm] [CodeGen] Ignore requiresStructuredCFG check in canSplitCriticalEdge if successor is loop header (PR #154063)

[Matt Arsenault via llvm-commits]

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@@ -1403,7 +1403,14 @@ bool MachineBasicBlock::canSplitCriticalEdge(
   const MachineFunction *MF = getParent();
   // Performance might be harmed on HW that implements branching using exec mask
   // where both sides of the branches are always executed.
-  if (MF->getTarget().requiresStructuredCFG())
+  // However, if `Succ` is a loop header, splitting the critical edge will not
+  // break structured CFG.
+  bool SuccIsLoopHeader = false;
+  if (MLI) {
+    const MachineLoop *L = MLI->getLoopFor(Succ);
+    SuccIsLoopHeader = L && L->getHeader() == Succ;
+  }
+  if (MF->getTarget().requiresStructuredCFG() && !SuccIsLoopHeader)
     return false;
----------------
arsenm wrote:

```suggestion

  if (MF->getTarget().requiresStructuredCFG()) {
    // If `Succ` is a loop header, splitting the critical edge will not
    // break structured CFG.
    if (MLI) {
       const MachineLoop *L = MLI->getLoopFor(Succ);
       return L && L->getHeader() == Succ;
    }

    return false;
  }
```

https://github.com/llvm/llvm-project/pull/154063