nir/large_constants: Handle incomplete derefs

nir/large_constants: Handle incomplete derefs

This pass works entirely with variables, all we have to do is ignore any
derefs we see which we can't chase back to the variable.  The one
interesting case we have to handle is if we have a complex use of a
deref_var.  In that case, we have to flag it non-constant.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6210>