cmd/5g: avoid temporary during OMINUS

Saves one MOVW and one register during the fast div/mod introduced in CL 6819123.

linux/arm (armv5)

benchmark               old ns/op    new ns/op    delta
BenchmarkInt64Mod1             12           12   +7.50%
BenchmarkUint16Mod2             7            7   +0.28%
BenchmarkUint16Mod4             7            7   -0.28%
BenchmarkUint64Mod1            15           11  -23.72%
BenchmarkInt8Neg                8            7  -17.66%
BenchmarkInt16Neg               8            7  -17.66%
BenchmarkInt32Neg               5            5   -9.04%
BenchmarkUint8Neg               7            6  -14.35%
BenchmarkUint16Neg              8            7  -17.66%
BenchmarkUint32Neg              5            5   -9.04%

R=rsc
CC=golang-dev
https://golang.org/cl/6842045

src/cmd/5g/cgen.c

@@ -15,7 +15,7 @@ void
 cgen(Node *n, Node *res)
 {
 	Node *nl, *nr, *r;
-	Node n1, n2, n3, f0, f1;
+	Node n1, n2, f0, f1;
 	int a, w, rg;
 	Prog *p1, *p2, *p3;
 	Addr addr;
@@ -240,13 +240,10 @@ cgen(Node *n, Node *res)
 	case OMINUS:
 		regalloc(&n1, nl->type, N);
 		cgen(nl, &n1);
-		nodconst(&n3, nl->type, 0);
-		regalloc(&n2, nl->type, res);
-		gmove(&n3, &n2);
-		gins(optoas(OSUB, nl->type), &n1, &n2);
-		gmove(&n2, res);
+		nodconst(&n2, nl->type, 0);
+		gins(optoas(OMINUS, nl->type), &n2, &n1);
+		gmove(&n1, res);
 		regfree(&n1);
-		regfree(&n2);
 		goto ret;
 
 	// symmetric binary

src/cmd/5g/gsubr.c

@@ -1611,6 +1611,16 @@ optoas(int op, Type *t)
 		a = ASUBD;
 		break;
 
+	case CASE(OMINUS, TINT8):
+	case CASE(OMINUS, TUINT8):
+	case CASE(OMINUS, TINT16):
+	case CASE(OMINUS, TUINT16):
+	case CASE(OMINUS, TINT32):
+	case CASE(OMINUS, TUINT32):
+	case CASE(OMINUS, TPTR32):
+		a = ARSB;
+		break;
+
 	case CASE(OAND, TINT8):
 	case CASE(OAND, TUINT8):
 	case CASE(OAND, TINT16):