Smis can easily be handled outside the stub call without adding much to code
size.

The ToString inlining adds overhead of repeated instance type loads and checks,
but under the assumption that it is called with mostly string values it should
speed things up (a local RegExp.p[@@replace] microbenchmark shows consistent
1.6% improvements).

Drive-by-fix: Remove duplication in ToString implementations.

BUG=

Review-Url: https://codereview.chromium.org/2874423003
Cr-Commit-Position: refs/heads/master@{#45287}