cmd/compile: consolidate alg code

cmd/compile: consolidate alg code

Pull all alg-related code into its own file.
subr.go is a Hobbesian Leviathan.

100% code movement. Cleanup and improvements to follow.

Change-Id: Ib9c8f66563fdda90c6e8cf646d366a9487a4648d
Reviewed-on: https://go-review.googlesource.com/19980
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>