The instruction size in bytes is now kInstrSize on all platforms.

Bug: v8:6666
Change-Id: I2f9880a6a74199a439c8327a4117efb74240aa22
Reviewed-on: https://chromium-review.googlesource.com/1164955
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54944}