[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://codereview.appspot.com/172850043