Change the keyword/identifier scan to a single loop that branchlessly
collects information on whether this is a possible keyword, identifier
terminator or slow path (i.e. escapes) by looking up the value in a
flags table (as long as the character is ascii).

Also rewrites that loop as an AdvanceUntil, and sprinkles in some
V8_LIKELY magic which is 'likely' to improve things.

Change-Id: If06b0fff23630e7593b515308e5ffeca2d65daa8
Reviewed-on: https://chromium-review.googlesource.com/c/1328943Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57391}