[dev.ssa] cmd/compile: use 2-result divide op

[dev.ssa] cmd/compile: use 2-result divide op

We now allow Values to have 2 outputs.  Use that ability for amd64.
This allows x,y := a/b,a%b to use just a single divide instruction.

Update #6815

Change-Id: Id70bcd20188a2dd8445e631a11d11f60991921e4
Reviewed-on: https://go-review.googlesource.com/25004
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>