nir/lower_io: Add support for nir_var_mem_constant

This commit adds support for nir_var_mem_constant various places.  It
also adds a pass similar to nir_lower_vars_to_explicit_types except it
also scrapes out the constants and stuffs them into constant_data.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6379>
3 files changed