[InstCombine] isFreeToInvert(): constant expressions aren't free to invert (PR50370)

This fixes https://bugs.llvm.org/show_bug.cgi?id=50370,
which reports a yet another endless combine loop,
this one regressed from 554b1bce,
which fixed yet another endless combine loop (PR50308)

This code had fallen into the very typical pitfall of forgetting
that constant expressions exist, and they aren't free to invert,
because the `not` won't be absorbed by the "constant",
but will remain a (constant) expression...