nir/loop_analyze: Refactor detection of limit vars

This commit reworks both get_induction_and_limit_vars() and
try_find_trip_count_vars_in_iand to return true on success and not
modify their output parameters on failure.  This makes their callers
significantly simpler.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 0333649e)