tree-optimization/116850 - corrupt post-dom info

tree-optimization/116850 - corrupt post-dom info

Path isolation computes post-dominators on demand but can end up
splitting blocks after that, wrecking it.  We can delay splitting
of blocks until we no longer need the post-dom info which is what
the following patch does to solve the issue.

	PR tree-optimization/116850
	* gimple-ssa-isolate-paths.cc (bb_split_points): New global.
	(insert_trap): Delay BB splitting if post-doms are computed.
	(find_explicit_erroneous_behavior): Process delayed BB
	splitting after releasing post dominators.
	(gimple_ssa_isolate_erroneous_paths): Do not free post-dom
	info here.

	* gcc.dg/pr116850.c: New testcase.