[MLIR][CF] Fix canonicalizer regression with 1-block cycle (#159743)

I caused a regression in flang due to my previous commits, since I
did not consider encountering a single-block cycle. I updated my
check to look for 1-block cycles in addition to any larger cycles
containing the successor block.

Author: benwu25

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

@@ -123,14 +123,16 @@ static LogicalResult collapseBranch(Block *&successor,
   if (successorDest == successor)
     return failure();
   // Don't try to collapse branches which participate in a cycle.
-  BranchOp nextBranch = dyn_cast<BranchOp>(successorDest->getTerminator());
+  Block *currBlock = successorDest;
+  BranchOp nextBranch = dyn_cast<BranchOp>(currBlock->getTerminator());
   while (nextBranch) {
     Block *nextBranchDest = nextBranch.getDest();
-    if (!nextBranchDest)
+    if (!nextBranchDest || nextBranchDest == currBlock)
       break;
     else if (nextBranchDest == successor)
       return failure();
-    nextBranch = dyn_cast<BranchOp>(nextBranchDest->getTerminator());
+    currBlock = nextBranchDest;
+    nextBranch = dyn_cast<BranchOp>(currBlock->getTerminator());
   }
 
   // Update the operands to the successor. If the branch parent has no

mlir/test/Transforms/control-flow-cycles.mlir

@@ -79,3 +79,15 @@ func.func @delayed_3_cycle() {
   ^bb5:
     cf.br ^bb3
 }
+
+// CHECK-LABEL:   @cycle_1_block
+// CHECK:           cf.br ^bb1
+// CHECK:         ^bb1:
+// CHECK:           cf.br ^bb1
+func.func @cycle_1_block() {
+  cf.br ^bb1
+  ^bb1:
+    cf.br ^bb2
+  ^bb2:
+    cf.br ^bb2
+}