Avoid repeating calculations in threader.

Avoid repeating calculations in threader.

We already attempt to resolve the current path on entry to
find_paths_to_name(), so there's no need to do so again for each
exported range since nothing has changed.

Removing this redundant calculation avoids 22% of calls into the path
solver.

Tested on x86-64 and ppc64le Linux with the usual regstrap.  I also
verified that the before and after number of threads was the same
in a suite of .ii files from a bootstrap.

gcc/ChangeLog:

	PR tree-optimization/102943
	* tree-ssa-threadbackward.c (back_threader::find_paths_to_names):
	Avoid duplicate calculation of paths.