[KnownBits] Add operator==

Checking whether two KnownBits are the same is somewhat common,
mainly in test code.

I don't think there is a lot of room for confusion with "determine
what the KnownBits for an icmp eq would be", as that has a
different result type (this is what the eq() method implements,
which returns Optional<bool>).

Differential Revision: https://reviews.llvm.org/D125692