Add a getAddressSpace method to the GEP instruction to mirror that of

Add a getAddressSpace method to the GEP instruction to mirror that of
the inttoptr instruction. The conceptual model here is that
'getAddressSpace' refers to the address space of this instruction's
type. It just happens that for GEPs, that is always the same as the
pointer operand's address space. We want both names so that access
patterns can be consistent between different instruction types.

llvm-svn: 167229