glsl: Check for SSBO variable in SSBO atomic lowering

glsl: Check for SSBO variable in SSBO atomic lowering

When an atomic function is called, we need to check to see if it is
for an SSBO variable before lowering it to the SSBO specific intrinsic
function.

v2:
 * is_in_buffer_block => is_in_shader_storage_block (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>