Suppress strict overflow optimization warnings.

This warning is emitted when the compiler performs an optimization, and wants
you to verify that the presumptions it is making are valid. For this case
we can correctly promise the compiler that the values will not overflow and
the optimizations are correct.

vtkm/worklet/contourtree/Mesh2D_DEM_VertexStarter.h

@@ -123,6 +123,15 @@ public:
   {
   }
 
+// For numerous calls of this function GCC is able to determine if i is
+// always greater than j ( or vice-versa ) and optimizes those call sites.
+// But when it does these optimizations is presumes that i and j will not
+// overflow and emits a Wstrict-overflow warning
+#ifdef VTKM_GCC
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-overflow"
+#endif
+
   // Locate the next vertex in direction indicated
   template <typename InFieldPortalType>
   VTKM_EXEC void operator()(const vtkm::Id& vertex,
@@ -213,7 +222,12 @@ public:
     linkMask = mask;
     chain = destination;
   } // operator()
-};  // Mesh2D_DEM_VertexStarter
+
+#ifdef VTKM_GCC
+#pragma GCC diagnostic pop
+#endif
+
+}; // Mesh2D_DEM_VertexStarter
 }
 }
 }