Fix discriminators of gimple PHI arguments

Fix discriminators of gimple PHI arguments

while gimple PHI itself does not have locations, the arguments does and they
can be used to determine count of edges in CFG.  Assign_discriminators already
knows how to handle goto_locus and PHI args should be handled same way as
done by this patch.

Bootstrapped/regtested x86_64-linux, comitted.

gcc/ChangeLog:

	* auto-profile.cc (function_instance::match): Sanity check
	that gimple PHI has no location.
	* tree-cfg.cc (assign_discriminators): Also remap locations
	of gimple PHI arguments.