Return VARYING in range_on_path_entry if nothing found.

Return VARYING in range_on_path_entry if nothing found.

The problem here is that the solver's code solving unknown SSAs on entry
to a path was returning UNDEFINED if there were no incoming edges to the
start of the path that were not the function entry block.  This caused a
cascade of pain down stream.

Tested on x86-64 Linux.

	PR tree-optimization/102511

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::range_on_path_entry):
	Return VARYING when nothing found.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr102511.c: New test.
	* gcc.dg/tree-ssa/ssa-dom-thread-14.c: Adjust.