This CL adds the simplified operators
  StringCodePointAt
  SeqStringCodePointAt
and associated lowering.

Bug: v8:7270
Change-Id: I5aeefddeec39c3119b2d225e92a3116f802e7b45
Reviewed-on: https://chromium-review.googlesource.com/861789
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50547}