nir: rename variables in nir_lower_io_to_temporaries for clarity

nir: rename variables in nir_lower_io_to_temporaries for clarity

In the emit_copies() function, the use of "newv" and "temp" names made
sense when only copies from temporaries to the new variables were
being done. But now there are other calls to copy with other pairings,
and "temp" doesn't always refer to a temporary created in this
pass. Use the names "dest" and "src" instead.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>