... by using KeyedStoreIC_Slow builtin instead. The issue with hard-coded
language mode is that the stub can be re-used through megamorphic stub cache for
an IC with incompatible language mode. KeyedStoreIC_Slow already does the
right thing - it decodes the language mode from the IC slot kind.

This CL also fixes the code kinds of the slow IC handlers. The code kind of
IC handlers is used only for checking that the handler was added to the right
megamorphic stub cache, which expect the handlers' code kinds to be either
Code::LOAD_IC or Code::STORE_IC.

And the megamorphic builtins are just helper code stubs that are called from
IC dispatchers, therefore they should have BUILTIN code kind. Same applies to
the other stubs which are neither IC dispatchers nor handlers.

BUG=v8:5917

Review-Url: https://codereview.chromium.org/2677603004
Cr-Commit-Position: refs/heads/master@{#42958}