radeonsi: emit s_waitcnt for shader memory barriers and volatile
radeonsi: emit s_waitcnt for shader memory barriers and volatile

Turns out that this is needed after all to satisfy some strengthened
coherency tests. Depends on support in LLVM, added in r267729.

v2: updated to reflect changes to the LLVM intrinsic

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com> (v1)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler