[InstCombine] Fold `(~x) | y` --> `~(x & (~y))` iff it is free to do so

Iff we know we can get rid of the inversions in the new pattern,
we can thus get rid of the inversion in the old pattern,
this decreasing instruction count.

Note that we could position this transformation as just hoisting
of the `not` (still, iff y is freely negatible), but the test changes
show a number of regressions, so let's not do that.