Remove old VRP jump threader code.

Remove old VRP jump threader code.

There's a lot of code that melts away without the ASSERT_EXPR based jump
threader.  Also, I cleaned up the include files as part of the process.

gcc/ChangeLog:

	* tree-vrp.c (lhs_of_dominating_assert): Remove.
	(class vrp_jt_state): Remove.
	(class vrp_jt_simplifier): Remove.
	(vrp_jt_simplifier::simplify): Remove.
	(class vrp_jump_threader): Remove.
	(vrp_jump_threader::vrp_jump_threader): Remove.
	(vrp_jump_threader::~vrp_jump_threader): Remove.
	(vrp_jump_threader::before_dom_children): Remove.
	(vrp_jump_threader::after_dom_children): Remove.