Implement vload_half{,n} and vload(half)

v2: add vload(half) as well
    make helpers amdgpu specific (NVPTX uses different private AS numbering)
    use clang builtin on clang >= 6

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tstellar@redhat.com>
llvm-svn: 312839