[wasm] Added DCHECKs that the input of the Int64Lowering is not null.

src/compiler/int64-lowering.cc

Index: src/compiler/int64-lowering.cc
diff --git a/src/compiler/int64-lowering.cc b/src/compiler/int64-lowering.cc
index 830a0de607af27e0d0ee06ab056f2742089d4d39..71f500886b40afe101ff6a32ed570ead578d0bfa 100644
--- a/src/compiler/int64-lowering.cc
+++ b/src/compiler/int64-lowering.cc
@@ -32,6 +32,8 @@ Int64Lowering::Int64Lowering(Graph* graph, MachineOperatorBuilder* machine,
       signature_(signature),
       placeholder_(graph->NewNode(common->Parameter(-2, "placeholder"),
                                   graph->start())) {
+  DCHECK_NOT_NULL(graph);
+  DCHECK_NOT_NULL(graph->end());
   replacements_ = zone->NewArray<Replacement>(graph->NodeCount());
   memset(replacements_, 0, sizeof(Replacement) * graph->NodeCount());
 }