[crankshaft] Enable ES6 instanceof optimization.

Note that this optimization only kicks in with --harmony-instanceof
being enabled as well. By itself this optimization itself does not
trigger.

R=bmeurer@chromium.org
BUG=v8:4447
LOG=n

Review URL: https://codereview.chromium.org/1817043002

Cr-Commit-Position: refs/heads/master@{#34947}

src/flag-definitions.h

@@ -274,7 +274,7 @@ DEFINE_BOOL(track_fields, true, "track fields with only smi values")
 DEFINE_BOOL(track_double_fields, true, "track fields with double values")
 DEFINE_BOOL(track_heap_object_fields, true, "track fields with heap values")
 DEFINE_BOOL(track_computed_fields, true, "track computed boilerplate fields")
-DEFINE_BOOL(harmony_instanceof_opt, false, "optimize ES6 instanceof support")
+DEFINE_BOOL(harmony_instanceof_opt, true, "optimize ES6 instanceof support")
 DEFINE_IMPLICATION(track_double_fields, track_fields)
 DEFINE_IMPLICATION(track_heap_object_fields, track_fields)
 DEFINE_IMPLICATION(track_computed_fields, track_fields)