nir: calculate trip count for more loops

This adds support to loop analysis for loops where the induction
variable is compared to the result of min(variable, constant).

For example:

   for (int i = 0; i < imin(x, 4); i++)
      ...

We add a new bool to the loop terminator struct in order to
differentiate terminators with this exit condition.

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

src/compiler/nir/nir_opt_loop_unroll.c

@@ -827,7 +827,8 @@ process_loops(nir_shader *sh, nir_cf_node *cf_node, bool *has_nested_loop_out)
       } else {
          /* Attempt to unroll loops with two terminators. */
          unsigned num_lt = list_length(&loop->info->loop_terminator_list);
-         if (num_lt == 2) {
+         if (num_lt == 2 &&
+             !loop->info->limiting_terminator->exact_trip_count_unknown) {
             bool limiting_term_second = true;
             nir_loop_terminator *terminator =
                list_first_entry(&loop->info->loop_terminator_list,