This CL centralizes constants related to decoding from several places
into one place and makes it no longer necessary to include
wasm-opcodes.h for some simple constants.

R=clemensh@chromium.org

Bug: 
Change-Id: I53aa81e34167df467bc7455b717bf67083033943
Reviewed-on: https://chromium-review.googlesource.com/859764
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50503}