The spec for String.split() requires loading "separator[@@split]".
The KeyedLoadIC for this goes generic right away, but still requires
a runtime call. To avoid that, reshuffle checks in the generic stub
and do the primitive-string -> String-function translation inline.

Review-Url: https://codereview.chromium.org/2654893003
Cr-Commit-Position: refs/heads/master@{#42797}