This refactoring makes some parameters const, and does not rely on the
fact that inputs are passed via NodeVector. Both changes are needed for
an upcoming refactoring of the wasm decoder.

R=bbudge@chromium.org

Change-Id: Ifbd6185ae9ea5a0ef526c2fd695e13e3f14475f4
Reviewed-on: https://chromium-review.googlesource.com/571004Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46729}