nir: calculate trip count for more loops

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>