[dev.ssa] cmd/internal/ssa: SSA cleanups

Mostly suggested by Alan.
Convert Const* ops to just one Const op.
Use more of go/types.
Get rid of typers, all types must be specified explicitly.

Change-Id: Id4758f2b887d8a6888e88a7e047d97af55e34b62
Reviewed-on: https://go-review.googlesource.com/8110
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/internal/ssa/compile.go

@@ -79,7 +79,7 @@ var passOrder = map[string]string{
 	// regalloc requires all the values in a block to be scheduled
 	//"schedule": "regalloc",
 	// code generation requires register allocation
-	//"cgen":"regalloc",
+	//"regalloc": "cgen",
 }
 
 func init() {