8277212: GC accidentally cleans valid megamorphic vtable inline caches

Reviewed-by: eosterlund, pliden, coleenp, thartmann

Author: stefank

src/hotspot/share/code/compiledMethod.cpp

@@ -478,6 +478,10 @@ bool CompiledMethod::clean_ic_if_metadata_is_dead(CompiledIC *ic) {
       } else {
         ShouldNotReachHere();
       }
+    } else {
+      // This inline cache is a megamorphic vtable call. Those ICs never hold
+      // any Metadata and should therefore never be cleaned by this function.
+      return true;
     }
   }