Fix and re-enable R600 vload/vstore assembly

The assembly optimizations were making unsafe assumptions about which address
spaces had which identifiers.

Also, fix vload/vstore with 64-bit pointers. This was broken previously on
Radeon SI.

This version still only has assembly versions of int/uint 2/4/8/16 for global
loads and stores on R600, but it does it in a way that would be very easily
extended to private/local/constant and could also be handled easily on other
architectures.

v2: 1) Leave v[load|store]_impl.ll in generic/lib
    2) Remove vload_if.ll and vstore_if.ll interfaces
    3) Fix address+offset calculations
    3) Remove offset from assembly arg list
llvm-svn: 186416