[flang] Fix a warning

This patch fixes:

  flang/lib/Lower/Bridge.cpp:2128:10: error: unused variable 'result'
  [-Werror,-Wunused-variable]

Author: kazutakahirata

flang/lib/Lower/Bridge.cpp

@@ -2129,6 +2129,7 @@ class FirConverter : public Fortran::lower::AbstractConverter {
         toLocation(), *this, info.reduceOperatorList, reduceVars,
         reduceVarByRef, reductionDeclSymbols, info.reduceSymList);
     assert(result && "Failed to process `do concurrent` reductions");
+    (void)result;
 
     doConcurrentLoopOp.getReduceVarsMutable().assign(reduceVars);
     doConcurrentLoopOp.setReduceSymsAttr(