[dev.power64] 5g: fix mistaken bit-wise AND in regopt

Replace a bit-wise AND with a logical one.  This happened to
work before because bany returns 0 or 1, but the intent here
is clearly logical (and this makes 5g match with 6g and 8g).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/172850043

src/cmd/5g/reg.c

@@ -393,7 +393,7 @@ loop2:
 		for(z=0; z<BITS; z++)
 			bit.b[z] = (r->refahead.b[z] | r->calahead.b[z]) &
 			  ~(externs.b[z] | params.b[z] | addrs.b[z] | consts.b[z]);
-		if(bany(&bit) & !r->f.refset) {
+		if(bany(&bit) && !r->f.refset) {
 			// should never happen - all variables are preset
 			if(debug['w'])
 				print("%L: used and not set: %Q\n", r->f.prog->lineno, bit);