nir: add nir_num_variable_modes and nir_var_mem_push_const

These will be useful in the upcoming load/store vectorizer.

v11: rebase

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
2 files changed