Convert shift ops to also encode the size of the shift amount.

Change signed right shift from using CMOV to using bit twiddles.
It is a little bit better (5 instructions instead of 4, but fewer
bytes and slightly faster code).  It's also a bit faster than
the 4-instruction branch version, even with a very predictable
branch.  As tested on my machine, YMMV.

Implement OCOM while we are here.

Change-Id: I8ca12dd62fae5d626dc0e6da5d4bbd34fd9640d2
Reviewed-on: https://go-review.googlesource.com/12867Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>