New instcombine rule: max(~a,~b) -> ~min(a, b)

This case is interesting because ScalarEvolutionExpander lowers min(a,
b) as ~max(~a,~b).  I think the profitability heuristics can be made
more clever/aggressive, but this is a start.

Differential Revision: http://reviews.llvm.org/D7821

llvm-svn: 230285