fix handling of mapped and their location

fix handling of mapped and their location

So when we using the newly mapped location, we should check if
it is not unknown location and if so just use the original location.
Note this is a latent bug in remove_forwarder_block_with_phi code too.

This fixes gcc.dg/uninit-pr40635.c when doing more mergephi.

gcc/ChangeLog:

	* tree-cfg.cc (copy_phi_arg_into_existing_phi): Use the original location
	if the mapped location is unknown.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>