[llvm] [InstCombine] Improve handling of `not` and free inversion. (PR #66787)

[via llvm-commits]

goldsteinn wrote:

> LGTM with the last commit removed (the test coverage is quite insufficient around m_AddSubInvertable, and I don't particularly want this fold anyway).

I'd be happy to improve the coverage. Why are you opposed to the commit as a whole?

https://github.com/llvm/llvm-project/pull/66787