Clean up registering of paths in backwards threader.

Clean up registering of paths in backwards threader.

All callers to maybe_register_path() call find_taken_edge() beforehand
and pass the edge as an argument.  There's no reason to repeat this
at each call site.

This is a clean-up in preparation for some other enhancements to the
backwards threader.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (back_threader::maybe_register_path):
	Remove argument and call find_taken_edge.
	(back_threader::resolve_phi): Do not calculate taken edge before
	calling maybe_register_path.
	(back_threader::find_paths_to_names): Same.