Bake the bit width and signedness into opcodes.
Pro: Rewrite rules become easier.  Less chance for confusion.
Con: Lots more opcodes.

Let me know what you think.  I'm leaning towards this, but I could be
convinced otherwise if people think this is too ugly.

Update #11467

Change-Id: Icf1b894268cdf73515877bb123839800d97b9df9
Reviewed-on: https://go-review.googlesource.com/12362Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>