glsl: Add lowering pass for shared variable references

glsl: Add lowering pass for shared variable references

In this lowering pass, shared variables are decomposed into intrinsic
calls.

v2:
 * Send mem_ctx as a parameter (Iago)

v3:
 * Shared variables don't have an associated interface block (Iago)
 * Always use 430 packing (Iago)
 * Comment / whitespace cleanup (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>