spirv: Rework handling of spec constant workgroup size built-ins

spirv: Rework handling of spec constant workgroup size built-ins

Instead of handling it as part of the handling of constant instructions,
just stash the vtn_value when we see the decoration and handle it
explicitly later.  This will let us re-order handling of constant
instructions without breaking the Vulkan SPIR-V requirement that
decorating a specialization constant as the WorkgroupSize built-in
overrides the workgroup size set as an execution mode.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>