__clc_mem_fence and __clc_work_group_barrier function have two
parameters memory_scope and memory_order. The design allows the clc
functions to implement SPIR-V ControlBarrier and MemoryBarrier
functions in the future.
The default memory ordering in clc is set to __ATOMIC_SEQ_CST, which is
also the default and strongest ordering in OpenCL and C++.
OpenCL cl_mem_fence_flags parameter is converted to combination of
__MEMORY_SCOPE_DEVICE and __MEMORY_SCOPE_WRKGRP, which is passed to clc.
llvm-diff shows no change to nvptx64--nvidiacl.bc.
llvm-diff show a small change to amdgcn--amdhsa.bc and the number of
LLVM IR instruction is reduced by 1: https://alive2.llvm.org/ce/z/_Uhqvt
af16fc2e2a