Use width-and-signed-specific multiply opcodes.
Implement OMUL.
A few other cleanups.

Fixes #11467

Change-Id: Ib0fe80a1a9b7208dbb8a2b6b652a478847f5d244
Reviewed-on: https://go-review.googlesource.com/12540Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>