The comment in {CallRuntime} was misleading. The {rsi}/{esi} register
did not hold the instance, but the context instead. The generated code
was correct thought.

R=titzer@chromium.org

Change-Id: I18020a04ac75faedf7ad5e4b07cab27ae0aedae7
Reviewed-on: https://chromium-review.googlesource.com/1051232Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53088}