[InferAddressSpaces] Fix assertion about null pointer

InferAddressSpaces does not check address space in collectFlatAddressExpressions,
which causes values with non flat address space put into Postorder and causes
assertion in cloneValueWithNewAddressSpace.

This patch fixes assertion in OpenCL 2.0 conformance test generic_address_space
subtest for amdgcn target.

Differential Revision: https://reviews.llvm.org/D34991

llvm-svn: 307349