nir: add guess trip count support to loop analysis

nir: add guess trip count support to loop analysis

This detects an induction variable used as an array index to guess
the trip count of the loop. This enables us to do a partial
unroll of the loop, which can eventually result in the loop being
eliminated.

v2: check if the induction var is used to index more than a single
    array and if so get the size of the smallest array.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>