nir: Rename parallel_copy_copy to parallel_copy_entry and add a foreach macro

nir: Rename parallel_copy_copy to parallel_copy_entry and add a foreach macro

parallel_copy_copy was a silly name.  Also, things were getting long and
annoying, so I added a foreach macro.  For historical reasons, several of
the original iterations over parallel copy entries in from_ssa used the
_safe variants of the loop.  However, all of these no longer ever remove an
entry so it's ok to make them all use the normal iterator.

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