amd64 and 386 have rules to reduce multiplication by a positive power
of two, but a more general reduction (both for positive and negative
powers of two) is already performed by generic rules that were added
in CL 36323 to replace walkmul (see lines 166:173 in generic.rules).

The x86 and amd64 rules are never triggered during all.bash and can be
removed, reducing rules duplication.

The change also adds a few code generation tests for amd64 and 386.

Change-Id: I566d48186643bd722a4c0137fe94e513b8b20e36
Reviewed-on: https://go-review.googlesource.com/68450
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>