nir: Handle incomplete derefs in split_struct_vars

nir: Handle incomplete derefs in split_struct_vars

In split_var_list_structs where we initalize the splitting, we already
use get_complex_used_vars to avoid splitting any variables that have a
complex use.  However, we weren't actually handling the complex uses
properly in the case where we can't actually find the variable.

Fixes: f1cb3348f1 "nir/split_vars: Properly bail in the presence of ..."
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6332>