[InstCombine] Fold ~A - Min/Max(~A, O) -> Max/Min(A, ~O) - A

[InstCombine] Fold ~A - Min/Max(~A, O) -> Max/Min(A, ~O) - A

This is an attempt to get out of a local-minimum that instcombine currently
gets stuck in. We essentially combine two optimisations at once, ~a - ~b = b-a
and min(~a, ~b) = ~max(a, b), only doing the transform if the result is at
least neutral. This involves using IsFreeToInvert, which has been expanded a
little to include selects that can be easily inverted.

This is trying to fix PR35875, using the ideas from Sanjay. It is a large
improvement to one of our rgb to cmy kernels.

Differential Revision: https://reviews.llvm.org/D52177

llvm-svn: 343569