nir/register: Add a parent_instr field

nir/register: Add a parent_instr field

This adds a parent_instr field similar to the one for ssa_def.  The
difference here is that the parent_instr field on a nir_register can be
NULL if the register does not have a unique definition or if that
definition does not dominate all its uses.  We set this field in the
out-of-SSA pass so that backends can get SSA-like information even after
they have gone out of SSA.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>