Cleanup back_threader::find_path_to_names.

Cleanup back_threader::find_path_to_names.

The main path discovery function was due for a cleanup.  First,
there's a nagging goto and second, my bitmap use was sloppy.  Hopefully
this makes the code easier for others to read.

Regstrapped on x86-64 Linux.  I also made sure there were no difference
in the number of threads with this patch.

No functional changes.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (back_threader::find_paths_to_names):
	Remove gotos and other cleanups.