Fix PR33878: BasicAA incorrectly assumes different address spaces don't alias

Remove code that assumed that a nullptr of address space != 0 couldnt alias with a non-null pointer. This is incorrect, since nothing can be concluded about a null pointer in an address space != 0.
This code was written before address spaces were introduced

Differential Revision: https://reviews.llvm.org/D37518

llvm-svn: 312648