[OpenCL] Handle AddressSpaceConversion when target address space does not change.

In codegen different address spaces may be mapped to the same address
space for a target, e.g. in x86/x86-64 all address spaces are mapped
to 0. Therefore AddressSpaceConversion should be translated by
CreatePointerBitCastOrAddrSpaceCast instead of CreateAddrSpaceCast.

Differential Revision: http://reviews.llvm.org/D18713

llvm-svn: 266107