glsl: Replace atomic_ssbo and ssbo_atomic with atomic

glsl: Replace atomic_ssbo and ssbo_atomic with atomic

The atomic functions can also be used with shared variables in compute
shaders.

When lowering the intrinsic in lower_ubo_reference, we still create an
SSBO specific intrinsic since SSBO accesses can be indirectly
addressed, whereas all compute shader shared variable live in a single
shared variable area.

v2:
 * Also remove the _internal suffix from ssbo atomic intrinsic names (Iago)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>