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 change 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.
Fixes: 0c787d57e6 ("tu: increase maxPushConstantsSize to 256.")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17777>
6929ccedff