[InstCombine] use isFreeToInvert to generalize min/max with 'not'

This mimics the code for the corresponding cmp-select idiom.

This also prevents an infinite loop because isFreeToInvert
does not match constant expressions.

So this patch solves the same problem as D108814 and obsoletes
it, but my main motivation is to enhance the pattern matching
to allow more invertible ops. That change will be a follow-up
patch on top of this one.

Differential Revision: https://reviews.llvm.org/D109058