cmd/6g, cmd/8g: Allow optimization of return registers.

The peephole optimizer would keep hands off AX and X0 during returns, even though go doesn't return through registers.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/7030046

src/cmd/6g/peep.c

@@ -1157,8 +1157,6 @@ copyu(Prog *p, Adr *v, Adr *s)
 		return 0;
 
 	case ARET:	/* funny */
-		if(v->type == REGRET || v->type == FREGRET)
-			return 2;
 		if(s != A)
 			return 1;
 		return 3;

src/cmd/8g/peep.c

@@ -886,8 +886,6 @@ copyu(Prog *p, Adr *v, Adr *s)
 		return 0;
 
 	case ARET:	/* funny */
-		if(v->type == REGRET || v->type == FREGRET)
-			return 2;
 		if(s != A)
 			return 1;
 		return 3;