[mlir:bytecode] Only visit the all regions path if the op has regions

Zero region operations return true for both isBeforeAllRegions and
isAfterAllRegions when using WalkStage. The bytecode walk only
expects region holding operations in the after regions path, so
guard against that.

Author: River707

mlir/lib/Bytecode/Writer/IRNumbering.cpp

@@ -238,7 +238,7 @@ void IRNumberingState::computeGlobalNumberingState(Operation *rootOp) {
   SmallVector<StackState> opStack;
   rootOp->walk([&](Operation *op, const WalkStage &stage) {
     // After visiting all nested regions, we pop the operation from the stack.
-    if (stage.isAfterAllRegions()) {
+    if (op->getNumRegions() && stage.isAfterAllRegions()) {
       // If no non-isolated uses were found, we can safely mark this operation
       // as isolated from above.
       OperationNumbering *numbering = opStack.pop_back_val().numbering;