spirv: Add support for lowering workgroup access to offsets

spirv: Add support for lowering workgroup access to offsets

Before, we always left workgroup variables as shared nir_variables and
let the driver call nir_lower_io.  This adds an option to do the
lowering directly in spirv_to_nir.  To do this, we implicitly assign the
variables a std430 layout and then treat them like a UBO or SSBO and
immediately lower all the way to an offset.

As a side-effect, the spirv_to_nir pass now handles variable pointers
for workgroup variables.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>