Add a fastpath for when byteOffset is undefined, which is the common
case. We can just replace it with 0 and avoid the modulo checks. Also
add a smi-fastpath for byteOffset, which avoids calling stubs for
arithmetic when unnecessary.

BUG=chromium:701668,v8:5977

Change-Id: Id431dad46bf3796ef32ab465f6787bbebe83437c
Reviewed-on: https://chromium-review.googlesource.com/456502
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43933}