This cleans up and simplifyies handling the bytes followin an opcode
with little helper structs that will be useful in the interpreter and
already have been in keeping OpcodeArity and OpcodeLength up to date
with the decoder.

R=bradnelson@chromium.org, ahaas@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1664883002

Cr-Commit-Position: refs/heads/master@{#33723}