cmd/6g: fix undefined behavior in reg.c

Update issue 8527

Fixes, cmd/6g/reg.c:847:24: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

LGTM=minux, rsc
R=minux, rsc
CC=dvyukov, golang-codereviews
https://codereview.appspot.com/129290043