cmd/compile/internal/ssa: avoid string conversion in zcse

Some ssa.Type implementations fell through to gc.Tconv which generated
garbage to produce a string form of the Type.

name      old time/op    new time/op    delta
Template     405ms ± 7%     401ms ± 6%    ~     (p=0.478 n=20+20)
GoTypes      1.32s ± 1%     1.30s ± 2%  -1.27%  (p=0.000 n=19+20)
Compiler     6.07s ± 2%     6.03s ± 2%    ~     (p=0.121 n=20+20)

name      old alloc/op   new alloc/op   delta
Template    63.9MB ± 0%    63.7MB ± 0%  -0.21%  (p=0.000 n=19+20)
GoTypes      220MB ± 0%     219MB ± 0%  -0.21%  (p=0.000 n=20+20)
Compiler     966MB ± 0%     965MB ± 0%  -0.11%  (p=0.000 n=20+20)

name      old allocs/op  new allocs/op  delta
Template      708k ± 0%      701k ± 0%  -0.99%  (p=0.000 n=20+20)
GoTypes      2.20M ± 0%     2.17M ± 0%  -1.43%  (p=0.000 n=17+20)
Compiler     9.45M ± 0%     9.36M ± 0%  -0.91%  (p=0.000 n=20+20)

Change-Id: I5fcc30e0f76a823d1c301d4980b583d716a75ce3
Reviewed-on: https://go-review.googlesource.com/20844Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/zcse.go

@@ -17,7 +17,7 @@ func zcse(f *Func) {
 			v := b.Values[i]
 			next := true
 			if opcodeTable[v.Op].argLen == 0 {
-				key := vkey{v.Op, keyFor(v), v.Aux, typeStr(v)}
+				key := vkey{v.Op, keyFor(v), v.Aux, v.Type}
 				if vals[key] == nil {
 					vals[key] = v
 					if b != f.Entry {
@@ -46,7 +46,7 @@ func zcse(f *Func) {
 		for _, v := range b.Values {
 			for i, a := range v.Args {
 				if opcodeTable[a.Op].argLen == 0 {
-					key := vkey{a.Op, keyFor(a), a.Aux, typeStr(a)}
+					key := vkey{a.Op, keyFor(a), a.Aux, a.Type}
 					if rv, ok := vals[key]; ok {
 						v.SetArg(i, rv)
 					}
@@ -61,15 +61,7 @@ type vkey struct {
 	op Op
 	ai int64       // aux int
 	ax interface{} // aux
-	t  string      // type
-}
-
-// typeStr returns a string version of the type of v.
-func typeStr(v *Value) string {
-	if v.Type == nil {
-		return ""
-	}
-	return v.Type.String()
+	t  Type        // type
 }
 
 // keyFor returns the AuxInt portion of a  key structure uniquely identifying a