[flang][runtime] Fixed dimension offset computation for MayAlias.

flang/runtime/assign.cpp

static void MaximalByteOffsetRange(
   }
   int n{desc.raw().rank};
   for (int j{0}; j < n; ++j) {
     const auto &dim{desc.GetDimension(j)};
     auto extent{dim.Extent()};
     if (extent > 0) {
       auto sm{dim.ByteStride()};
       if (sm < 0) {
-        least += extent * sm;
+        least += (extent - 1) * sm;
       } else {
-        most += extent * sm;
+        most += (extent - 1) * sm;
       }
     }
   }
   most += desc.ElementBytes() - 1;
 }
 
 static inline bool RangesOverlap(const char *aStart, const char *aEnd,
     const char *bStart, const char *bEnd) {