tree-optimization/107087 - missed CCP after forwprop

tree-optimization/107087 - missed CCP after forwprop

When forwprop simplifies the CFG the 2nd order opportunities by
exposed degenerate PHIs are not realized.  The following improves
this by properly tracking executable edges and thus handling this
for non-cyclic CFGs at least.

This avoids the bogus diagnostic reported for the testcase in this PR.

	PR tree-optimization/107087
	* tree-ssa-forwprop.cc (pass_forwprop::execute): Track
	executable regions to avoid useless work and to better
	propagate degenerate PHIs.

	* g++.dg/pr107087.C: New testcase.