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/19980Reviewed-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>

src/cmd/compile/internal/gc/go.go

@@ -26,25 +26,6 @@ const (
 	MaxStackVarSize = 10 * 1024 * 1024
 )
 
-const (
-	// These values are known by runtime.
-	ANOEQ = iota
-	AMEM0
-	AMEM8
-	AMEM16
-	AMEM32
-	AMEM64
-	AMEM128
-	ASTRING
-	AINTER
-	ANILINTER
-	AFLOAT32
-	AFLOAT64
-	ACPLX64
-	ACPLX128
-	AMEM = 100
-)
-
 const (
 	// Maximum size in bits for Mpints before signalling
 	// overflow and also mantissa precision for Mpflts.