Adds support for high multiply which is used by the frontend when
rewriting const division.  The frontend currently only does this for 8,
16, and 32 bit integer arithmetic.

Change-Id: I9b6c6018f3be827a50ee6c185454ebc79b3094c8
Reviewed-on: https://go-review.googlesource.com/13696Reviewed-by: Keith Randall <khr@golang.org>