nir: Switch the arguments to nir_foreach_phi_src

nir: Switch the arguments to nir_foreach_phi_src

This matches the "foreach x in container" pattern found in many other
programming languages.  Generated by the following regular expression:

s/nir_foreach_phi_src(\([^,]*\),\s*\([^,]*\))/nir_foreach_phi_src(\2, \1)/

and a similar expression for nir_foreach_phi_src_safe.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>