nir/loop_analyze: Don't test non-positive iterations count

nir/loop_analyze: Don't test non-positive iterations count

Testing negative iterations count makes no sense, and can cause issues
when the unsigned type is used.
Testing 0 iterations is already covered with
will_break_on_first_iteration, so it can be skipped too.

Fixes: 6772a17a ("nir: Add a loop analysis pass")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9913
Signed-off-by: Sviatoslav Peleshko <sviatoslav.peleshko@globallogic.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26173>
(cherry picked from commit a6459e0f7bcb662130c79764ccf54ccfa9151182)