Add a sequential string type to the compiler, and transform
charCodeAt on SeqString into SeqStringCharCodeAt.

SeqStringCharCodeAt can handle one and two byte strings.

Bug: v8:6391
Change-Id: I2785257522c28f3b268c9833f5313e9630cb982a
Reviewed-on: https://chromium-review.googlesource.com/509573Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45508}