nir/loop_analyze: Handle bit sizes correctly in calculate_iterations

The current code assumes everything is 32-bit which is very likely true
but not guaranteed by any means.  Instead, use nir_eval_const_opcode to
do the calculations in a bit-size-agnostic way.  We also use the new
constant constructors to build the correct size constants.

Fixes: 6772a17acc8ee "nir: Add a loop analysis pass"
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>