This adds a new simplified operator NumberToString, which just lowers to
a call to the NumberToString builtin, and hooks that up to the typed
lowering (addressing a long-standing TODO).

Drive-by-fix: Also remove the %NumberToString runtime entry, and just
always use the %NumberToStringSkipCache entry from CSA, since we only
go there if the cache lookup already failed.

Bug: v8:5267, v8:7109
Change-Id: I5ca698c98679653813088a404f1fd38903a73c0e
Reviewed-on: https://chromium-review.googlesource.com/779099
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50636}