A minor refactoring of the decoder interface, which makes implementing
the baseline compiler easier.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ia5ae66e0e036329767b6e4f1cfcd3ed6a5e4cb74
Reviewed-on: https://chromium-review.googlesource.com/715636
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48498}