The CheckSeqString operator unnecessarily materialized two bits for the
separate conditions, but we can actually check for SeqString with just a
single mask instead, which saves the intermediate computations and two
registers.

Bug: v8:5267
Change-Id: Ib937a5a4eebdc271a2bbbc6f074f5992e06b2fa9
Reviewed-on: https://chromium-review.googlesource.com/741702Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49023}