nir: Add a foreach_ssa_def function

nir: Add a foreach_ssa_def function

There are some functions whose destinations are SSA-only and so aren't a
nir_dest.  This provides a function that is capable of iterating over the
SSA definitions defined by those functions.  If you want registers, you
should use the old iterator.

v2: Kenneth Graunke <kenneth@whitecape.org>:
 - Fix nir_foreach_ssa_def's return value.

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