This is a reland of 3ff4b447.

Original version did not handle V8_INTL_SUPPORT.

Original change's description:
> [turbofan] Move String.* functions to JSCallReducer
>
> Bug: v8:7250, v8:7340
> Change-Id: Ibb8d5badf89c66bd9bcb6bb390256ae81d0e899c
> Reviewed-on: https://chromium-review.googlesource.com/913208
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51505}

Bug: v8:7250, v8:7340
Change-Id: Id908cbcfaa9e9cf5459d6d3289e6ec00e387d287
Reviewed-on: https://chromium-review.googlesource.com/934268Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51514}