nir/dead_variables: Respect the modes passed to remove_dead_vars

nir/dead_variables: Respect the modes passed to remove_dead_vars

For the most part, this doesn't actually matter today.  We already only
call remove_dead_vars on the lists that are specified in the modes.  The
only functional change here is for the uniform, mem_ubo, and mem_ssbo
modes because they share a list.  If nir_remove_dead_variables is called
with a mode of nir_var_uniform, it will no longer remove UBOs or SSBOs,
for instance.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5966>