[InstCombine] add one-use check for min/max fold with not operands; NFC

This makes the intrinsic logic match the cmp+select idiom folds
just below. It's not clearly a win either way unless we think
that a 'not' op costs more than min/max.

The cmp+select folds on these patterns are more extensive than
the intrinsics currently and may have some complicated interactions,
so I'm trying to make those line up and bring the optimizations
for intrinsics up to parity.