glsl: lower SSBO atomic intrinsics

glsl: lower SSBO atomic intrinsics

The first argument to SSBO atomics is a reference to a SSBO buffer variable
so we want to compute its block index and offset and provide these values
to an internal version of the intrinsic that takes them instead of the
buffer variable reference.

v2:
- Support single components of integer vectors to be passed in as arguments.
- Get interface packing information from interface's type.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>