Add the max arg length to opcodes and use it in zcse.  Doesn't affect
speed, but allows better checking in checkFunc and removes the need
to keep a list of zero arg opcodes up to date.

Change-Id: I157c6587154604119720ec6228b767b6e52bb5c7
Reviewed-on: https://go-review.googlesource.com/19994Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>