tree-optimization/106809 - compile time hog in VN

tree-optimization/106809 - compile time hog in VN

The dominated_by_p_w_unex function is prone to high compile time.
With GCC 12 we introduced a VN run for uninit diagnostics which now
runs into a degenerate case with bison generated code.  Fortunately
this case is easy to fix with a simple extra check - a more
general fix needs more work.

	PR tree-optimization/106809
	* tree-ssa-sccvn.cc (dominaged_by_p_w_unex): Check we have
	more than one successor before doing extra work.

	* gcc.dg/torture/pr106809.c: New testcase.