Code comments help a lot to understand the generated code. Add a
comment before each instruction, and some special comments for longer
instructions.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ic18974e5cc89e23533e3abc54b0389723b77ff73
Reviewed-on: https://chromium-review.googlesource.com/1049626
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53064}