Fix uninitialized variable compiler errors [GCC 4.8.4]

R=svenpanne@chromium.org, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1156293003

Cr-Commit-Position: refs/heads/master@{#28851}

src/compiler/ast-graph-builder.cc

@@ -2216,13 +2216,12 @@ void AstGraphBuilder::VisitThrow(Throw* expr) {
 
 
 void AstGraphBuilder::VisitProperty(Property* expr) {
-  Node* value;
+  Node* value = nullptr;
   LhsKind property_kind = Property::GetAssignType(expr);
   VectorSlotPair pair = CreateVectorSlotPair(expr->PropertyFeedbackSlot());
   switch (property_kind) {
     case VARIABLE:
       UNREACHABLE();
-      value = nullptr;
       break;
     case NAMED_PROPERTY: {
       VisitForValue(expr->obj());