[MLIR][CF] Do not bail out early in collapseBranch to avoid regression (#159743)

Bailing out as soon as a cycle is detected causes a regression in flang, where
the test expects more optimizations than we allowed on an infinite loop. We
may be able to adjust this testcase.

Author: benwu25

mlir/lib/Dialect/ControlFlow/IR/ControlFlowOps.cpp

@@ -124,11 +124,13 @@ static LogicalResult collapseBranch(Block *&successor,
     return failure();
   // Don't try to collapse branches which participate in a cycle.
   BranchOp nextBranch = dyn_cast<BranchOp>(successorDest->getTerminator());
-  llvm::DenseSet<Block *> visited{successor, successorDest};
+  llvm::DenseSet<Block *> visited{successorDest};
   while (nextBranch) {
     Block *nextBranchDest = nextBranch.getDest();
-    if (visited.contains(nextBranchDest))
+    if (nextBranchDest == successor)
       return failure();
+    else if (visited.contains(nextBranchDest))
+      break;
     visited.insert(nextBranchDest);
     nextBranch = dyn_cast<BranchOp>(nextBranchDest->getTerminator());
   }