[dev.ssa] cmd/compile/internal/ssa: Fix bootstrap of ssa codegen

The cmd/compile/internal/ssa/gen directory can't depend on cmd/internal/gc
because that package doesn't exist in go1.4.  Use strings instead of
constants from that package.

The asm fields seem somewhat redundant to the opcode names we
conventionally use.  Maybe we can just trim the lowercase from the end
of the op name?  At least by default?

Change-Id: I96e8cda44833763951709e2721588fbd34580989
Reviewed-on: https://go-review.googlesource.com/11129
Reviewed-by: Michael Matloob <michaelmatloob@gmail.com>

src/cmd/compile/internal/ssa/gen/main.go

@@ -9,8 +9,6 @@ package main
 
 import (
 	"bytes"
-	"cmd/internal/obj"
-	"cmd/internal/obj/x86"
 	"fmt"
 	"go/format"
 	"io/ioutil"
@@ -27,7 +25,7 @@ type arch struct {
 type opData struct {
 	name string
 	reg  regInfo
-	asm  int16
+	asm  string
 }
 
 type blockData struct {
@@ -114,8 +112,8 @@ func genOp() {
 		for _, v := range a.ops {
 			fmt.Fprintln(w, "{")
 			fmt.Fprintf(w, "name:\"%s\",\n", v.name)
-			if v.asm != 0 {
-				fmt.Fprintf(w, "asm: x86.A%s,\n", x86.Anames[v.asm-obj.ABaseAMD64])
+			if v.asm != "" {
+				fmt.Fprintf(w, "asm: x86.A%s,\n", v.asm)
 			}
 			fmt.Fprintln(w, "reg:regInfo{")
 			fmt.Fprintln(w, "inputs: []regMask{")