[MLIR] Fix SCF verifier crash (llvm#153974)

An operand of the nested yield op can be null and hasn't been verified
yet when processing the enclosing operation. Using `getResultTypes()`
will dereference this null Value and crash in the verifier.

Author: joker-eph

mlir/lib/Dialect/SCF/IR/SCF.cpp

@@ -4236,14 +4236,15 @@ LogicalResult scf::IndexSwitchOp::verify() {
              << "see yield operation here";
     }
     for (auto [idx, result, operand] :
-         llvm::zip(llvm::seq<unsigned>(0, getNumResults()), getResultTypes(),
-                   yield.getOperandTypes())) {
-      if (result == operand)
+         llvm::enumerate(getResultTypes(), yield.getOperands())) {
+      if (!operand)
+        return yield.emitOpError() << "operand " << idx << " is null\n";
+      if (result == operand.getType())
         continue;
       return (emitOpError("expected result #")
               << idx << " of each region to be " << result)
                  .attachNote(yield.getLoc())
-             << name << " returns " << operand << " here";
+             << name << " returns " << operand.getType() << " here";
     }
     return success();
   };