Rename nir_lower_constant_initializers to nir_lower_variable_initalizers

This is naming is more clear as nir_variables can be initializes not
just with a nir_constant but with a pointer to another nir_variable.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3047>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3047>
12 files changed