[InstCombine] prefer xor with -1 because 'not' is easier to understand (PR32706)

This is a retry of rL300977 which was reverted because of infinite loops.
We have fixed all of the known places where that would happen, but there's
still a chance that this patch will cause infinite loops.

This matches the demanded bits behavior in the DAG and should fix:
https://bugs.llvm.org/show_bug.cgi?id=32706

Differential Revision: https://reviews.llvm.org/D32255