turnip: minor cleanups for push constants

Remove shared_consts from tu_compiled_shaders. Skip gather_push_constants when shared consts are enabled. This makes sure push_consts is only zero-initialized, and reserved_user_consts is 0. This saves some space in the const file.

This also adds a few asserts and a comment to lower_load_push_constant. Because shared consts share the same range for all stages, we should not apply per-stage offsets in lower_load_push_constant. It worked because nir_lower_explicit_io always sets base to 0 for nir_var_mem_push_const and shader->push_consts.lo was always 0 for all stages.