intprops: remove assumption about A|B representation

intprops: remove assumption about A|B representation

* lib/intprops.h (_GL_BINARY_OP_OVERFLOW): Do not assume that A|B
is a valid integer if both A and B are.  Although this is true for
all known practical hosts, the C standard doesn't guarantee it,
and the code need not assume it.  Also, this change may work around
HP-UX 11.23 and IRIX 6.5 cc bugs reported by Bruno Haible in
<http://lists.gnu.org/archive/html/bug-gnulib/2011-05/msg00426.html>.