isl_stream_next_token: treat "-0" as two tokens '-' and '0'

isl_stream_next_token: treat "-0" as two tokens '-' and '0'

Usually, a '-' followed by a sequence of digits is treated as a single
(negative) number.  When parsing affine expressions, negative numbers
are allowed without a preceding '+'.  However, if "-0" is parsed as '0'
then there is no way of distinguishing between "-0" and "0" and so "-0"
was not allowed in affine expressions.
We now treat "-0" as two tokens, so that it can be used in affine expressions.
The flip side is that "-0" can no longer be used where a single integer
is expected.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>