add new Min-replacement pattern to compute_cdf

If you don't do this you might get alternate results

https://travis-ci.org/smichr/sympy/jobs/19850479 shows the failure in
a run where 3 of the 4 python versions passed but one failed.
slight refactoring in MinMaxBase and XXX re canonical args
Merge pull request #2983 from smichr/pat

add new Min-replacement pattern to compute_cdf