[PATCH 3/4] read_mem_fence() support for R600 targets

This patch introduces three new intrinsics and therefore
  must be used in conjunction with the patches to the LLVM backend. It adds the
  read_mem_fence built-in to libclc, and an implementation for r600 using the
  newly introduced LLVM intrinsics. Plain copy&paste from the mem_fence patch,
  with some find&replace.

Signed-off-by: Damien Hilloulin <damien.hilloulin@supelec.fr>

This patch introduces three new intrinsics and therefore
must be used in conjunction with the patches to the LLVM backend.
It adds the
read_mem_fence built-in to libclc, and an implementation for r600
using the
newly introduced LLVM intrinsics. Plain copy&paste from the
mem_fence patch,
with some find&replace.

Same comments as patch #2. Split into two patches, generic implementation
then r600 implementation and drop the globallocal intrinsic.

-Tom