This CL further optimizes the decoding fastpath by moving feature
checks off the critical path. For prototype opcodes that are enabled
by feature flags, they are handled in a switch case off the main
path.

R=mstarzinger@chromium.org

Change-Id: If40fedbaadb9c611c78bc2b7df035ced056cb39a
Reviewed-on: https://chromium-review.googlesource.com/1076187Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53406}