This patch introduces three new intrinsics and therefore
must be used in conjunction with the patches to the LLVM backend. It adds the
write_mem_fence built-in to libclc, and an implementation for r600 using the
newly introduced LLVM intrinsics. Plain copy&paste from the read_mem_fence
patch, with some find&replace.
Signed-off-by: Damien Hilloulin <firstname.lastname@example.org>