The inbounds inference added by r277950 ("[InstCombine] Infer inbounds
on geps of allocas", 2016-08-07) could construct a new inbounds gep,
but ignore the address space.
Avoid asserting when the address space does not match. We could
introduce a new addrspacecast, but as one of the reviewers pointed
out, it is not clear that instcombine is allowed to introduce
addrspacecasts that weren't in the original program.