Simplify the instruction decoder.

The decoder for format 35mi was *mostly* identical to the one for
35c/35ms. The one place where it differed was in the allowing (or not)
of a fifth argument. I combined the two blocks and just added an
additional format check in the five-argument case. I ended up
rewriting the stuff to pull arguments into the args array, because
while the original loop at the core of it was attractively simple, the
extra stuff around it just made the code obscure. With this change,
the code just does switch(count) and has a nice little cascade.

While I was in the territory, I did a couple other minor cleanups.

Change-Id: Ib75d47eae92a91571445751789d56feeb4e0ca81