[InstSimplify] add folds for or-of-icmps with same operands

[InstSimplify] add folds for or-of-icmps with same operands

All of these (and a few more) are already handled by InstCombine,
but we shouldn't have to wait until then to simplify these because
they're cheap to deal with here in InstSimplify.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288833 91177308-0d34-0410-b5e6-96231b3b80d8