5c, 5g: fix build with too-smart gcc

R=ken2
CC=golang-dev
https://golang.org/cl/4543049

src/cmd/5c/peep.c

@@ -1100,7 +1100,7 @@ copyu(Prog *p, Adr *v, Adr *s)
 		if(v->type == D_REG) {
 			if(v->reg <= REGEXT && v->reg > exregoffset)
 				return 2;
-			if(v->reg == (uchar)REGARG)
+			if(v->reg == REGARG)
 				return 2;
 		}
 		if(v->type == D_FREG)
@@ -1118,7 +1118,7 @@ copyu(Prog *p, Adr *v, Adr *s)
 
 	case ATEXT:	/* funny */
 		if(v->type == D_REG)
-			if(v->reg == (uchar)REGARG)
+			if(v->reg == REGARG)
 				return 3;
 		return 0;
 	}

src/cmd/5g/peep.c

@@ -1134,7 +1134,7 @@ copyu(Prog *p, Adr *v, Adr *s)
 		if(v->type == D_REG) {
 			if(v->reg <= REGEXT && v->reg > exregoffset)
 				return 2;
-			if(v->reg == (uchar)REGARG)
+			if(v->reg == REGARG)
 				return 2;
 		}
 		if(v->type == D_FREG)
@@ -1152,7 +1152,7 @@ copyu(Prog *p, Adr *v, Adr *s)
 
 	case ATEXT:	/* funny */
 		if(v->type == D_REG)
-			if(v->reg == (uchar)REGARG)
+			if(v->reg == REGARG)
 				return 3;
 		return 0;
 	}