Remove VRP threader.

Remove VRP threader.

Now that things have stabilized, we can remove the old code.

I have left the hybrid threader in tree-ssa-threadedge, even though the
VRP threader was the only user, because we may need it as an interim
step for DOM threading removal.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-pass.h (make_pass_vrp_threader): Remove.
	* tree-ssa-threadbackward.c
	(back_threader_profitability::profitable_path_p): Remove
	ASSERT_EXPR references.
	* tree-ssa-threadedge.c (jt_state::register_equivs_stmt): Same.
	* tree-vrp.c (vrp_folder::simplify_casted_conds): Same.
	(execute_vrp): Same.
	(class hybrid_threader): Remove.
	(hybrid_threader::hybrid_threader): Remove.
	(hybrid_threader::~hybrid_threader): Remove.
	(hybrid_threader::before_dom_children): Remove.
	(hybrid_threader::after_dom_children): Remove.
	(execute_vrp_threader): Remove.
	(class pass_vrp_threader): Remove.
	(make_pass_vrp_threader): Remove.