nir: Use nir_foreach_ssa_def for setting up ssa destinations

nir: Use nir_foreach_ssa_def for setting up ssa destinations

Before, we were using foreach_dest and switching on whether the destination
was an SSA value.  This works, except not all destinations are SSA values
so we have to special-case ssa_undef instructions.  Now that we have a
foreach_ssa_def function, we can iterate over all of the register
destinations in one pass and iterate over the SSA destinations in a second.
This way, if we add other ssa-only instructions, we won't have to worry
about adding them to the special case we have for ssa_undef.

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