Move copy_phi_arg_into_existing_phi to common location and use it

Move copy_phi_arg_into_existing_phi to common location and use it

While improving replace_phi_edge_with_variable for the diamond formed bb
case, I need a way to copy phi entries from one edge to another as I am
removing a forwarding bb inbetween. I was pointed out that jump threading
code had copy_phi_arg_into_existing_phi which I can use.
I also noticed that both gimple_duplicate_sese_tail and
remove_forwarder_block have similar code so it makes sense to use that function
in those two locations too.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* tree-ssa-threadupdate.cc (copy_phi_arg_into_existing_phi): Move to ...
	* tree-cfg.cc (copy_phi_arg_into_existing_phi): Here and remove static.
	(gimple_duplicate_sese_tail): Use copy_phi_arg_into_existing_phi instead
	of an inline version of it.
	* tree-cfgcleanup.cc (remove_forwarder_block): Likewise.
	* tree-cfg.h (copy_phi_arg_into_existing_phi): New declaration.