[flang] Fix a warning

This patch fixes:

  flang/lib/Optimizer/HLFIR/Transforms/LowerHLFIROrderedAssignments.cpp:1377:10:
  error: variable 'isValid' set but not used
  [-Werror,-Wunused-but-set-variable]
diff --git a/flang/lib/Optimizer/HLFIR/Transforms/LowerHLFIROrderedAssignments.cpp b/flang/lib/Optimizer/HLFIR/Transforms/LowerHLFIROrderedAssignments.cpp
index 89b5ccb..03cc92e 100644
--- a/flang/lib/Optimizer/HLFIR/Transforms/LowerHLFIROrderedAssignments.cpp
+++ b/flang/lib/Optimizer/HLFIR/Transforms/LowerHLFIROrderedAssignments.cpp
@@ -1397,6 +1397,7 @@
       isValid = false;
     }
     assert(isValid && "invalid live-in");
+    (void)isValid;
     return op;
   };
   mlir::Liveness liveness(whereOp.getOperation());