Allow vtable-style dispatch of abstract static methods

Summary: I left this out because of a weird measurement in perflab (see
D1366913), but apparently at the time got a clean run that doubled Func* lookup
for abstract static methods between vtable-style dispatch and LdObjMethod and
asserted they were the same Func*, without problems, so I'm a little surprised
I thought "more testing" was still needed to enable it.

Reviewed By: @ottoni

Differential Revision: D2093908

hphp/runtime/vm/jit/irgen-call.cpp

@@ -174,13 +174,7 @@ void fpushObjMethodCommon(IRGS& env,
         g_context->lookupObjMethod(func, baseClass, methodName, curClass(env),
                                    false);
       if (res == LookupResult::MethodFoundWithThis ||
-          /*
-           * TODO(#4455926): We don't allow vtable-style dispatch of
-           * abstract static methods, but not for any real reason
-           * here.  It should be able to work, but needs further
-           * testing to be enabled.
-           */
-          (res == LookupResult::MethodFoundNoThis && !func->isAbstract())) {
+          res == LookupResult::MethodFoundNoThis) {
         /*
          * If we found the func in baseClass, then either:
          *  a) its private, and this is always going to be the