tree-optimization/116460 - improve forwprop compile-time

The following improves forwprop block reachability which I noticed
when debugging PR116460 and what is also noted in the comment.  It
avoids processing blocks in natural loops determined unreachable,
thereby making the issue in PR116460 latent.

	PR tree-optimization/116460
	* tree-ssa-forwprop.cc (pass_forwprop::execute): Do not
	process blocks in unreachable natural loops.