8177346: hotspot change for 8176513 breaks jdk9 build on Ubuntu 16.04

Reviewed-by: dholmes, kvn, vlivanov

hotspot/src/share/vm/opto/library_call.cpp

@@ -2372,7 +2372,7 @@ bool LibraryCallKit::inline_unsafe_access(bool is_store, const BasicType type, c
   // the barriers get omitted and the unsafe reference begins to "pollute"
   // the alias analysis of the rest of the graph, either Compile::can_alias
   // or Compile::must_alias will throw a diagnostic assert.)
-  bool need_mem_bar;
+  bool need_mem_bar = false;
   switch (kind) {
       case Relaxed:
           need_mem_bar = mismatched && !adr_type->isa_aryptr();