nir/dead_write_vars: Get modes directly from derefs

Instead of going all the way back to the variable, just look at the
deref.  The modes are guaranteed to be the same by nir_validate whenever
the variable can be found.  This fixes clear_unused_for_modes for
derefs that don't have an accessible variable.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
1 file changed