New @test13 in Attributor/align.ll is the main motivation - null pointer
really does not limit our alignment knowledge, in fact it is fully aligned
since it has no bits set.
Here we don't special-case null pointer because it is somewhat controversial
to add one more place where we enforce that null pointer is zero,
but instead we do the more general thing of trying to perform constant-fold
of pointer constant to an integer, and perform alignment inferrment on that.