[InstCombine] Simplify 'xor' to 'or' if no common bits are set.

Summary:
In order to get the whole fold as specified in [[ https://bugs.llvm.org/show_bug.cgi?id=6773 | PR6773 ]],
let's first handle the simple straight-forward things.
Let's start with the `and` -> `or` simplification.

The one obvious thing missing here: the constant mask is not handled.
I have an idea how to handle it, but it will require some thinking,
and is not strictly required here, so i've left that for later.

https://rise4fun.com/Alive/Pkmg

Reviewers: spatel, craig.topper, eli.friedman, jingyue

Reviewed By: spatel

Subscribers: llvm-commits

Was reviewed as part of https://reviews.llvm.org/D45631

llvm-svn: 330103