tree-optimization/120003 - missed jump threading

The following allows the entry and exit block of a jump thread path
to be equal, which can easily happen when there isn't a forwarder
on the interesting edge for an FSM thread conditional.  We just
don't want to enlarge the path from such a block.

	PR tree-optimization/120003
	* tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
	Allow block re-use but do not enlarge the path beyond such a
	re-use.

	* gcc.dg/tree-ssa/ssa-thread-23.c: New testcase.
	* gcc.dg/tree-ssa/ssa-dom-thread-7.c: Adjust.

(cherry picked from commit 1a13684dfc7286139064f7d7341462c9995cbd1c)