Build error with Python 2.6.

  GEN    nir/nir_opt_algebraic.c
Traceback (most recent call last):
  File "./nir/nir_opt_algebraic.py", line 726, in <module>
    for left, right in itertools.combinations_with_replacement(invert.keys(), 2):
AttributeError: 'module' object has no attribute 'combinations_with_replacement'


commit d9ca4a172e8e8a42217588ca1b83a7ceb5bf3f76
Author: Mathieu Bridon <bochecha@daitauha.fr>
Date:   Thu Aug 9 10:27:19 2018 +0200

    python: Use the right function for the job
    
    The code was just reimplementing itertools.combinations_with_replacement
    in a less efficient way.
    
    This does change the order of the results slightly, but it should be ok.
    
    Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
    Reviewed-by: Dylan Baker <dylan@pnwbakers.com>