nir/metadata: Rename metadata_dirty to metadata_preserve

nir/metadata: Rename metadata_dirty to metadata_preserve

nir_metadata_dirty was a terrible name because the parameter it takes is
the metadata to be preserved.  This is really confusing because it looks
like it's doing the opposite of what it is actually doing.  Now it's named
sensibly.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>