cmd/compile: move constant divide strength reduction to SSA rules

cmd/compile: move constant divide strength reduction to SSA rules

Currently the conversion from constant divides to multiplies is mostly
done during the walk pass.  This is suboptimal because SSA can
determine that the value being divided by is constant more often
(e.g. after inlining).

Change-Id: If1a9b993edd71be37396b9167f77da271966f85f
Reviewed-on: https://go-review.googlesource.com/37015
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>