remove SLP_TREE_TWO_OPERATORS, add SLP permutation node

remove SLP_TREE_TWO_OPERATORS, add SLP permutation node

This removes the SLP_TREE_TWO_OPERATORS hack in favor of having
explicit SLP nodes for both computations and the blend operation.
For this introduce a generic merge + select + permute SLP node
(with implementation limits).

Building upon earlier patches it adds vect_stmt_dominates_stmt_p
and the ability to compute a vector insertion place from
vectorized stmts (which now have UID zero) as needed for
the permute node.

2020-06-17  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (_slp_tree::two_operators): Remove.
	(_slp_tree::lane_permutation): New member.
	(_slp_tree::code): Likewise.
	(SLP_TREE_TWO_OPERATORS): Remove.
	(SLP_TREE_LANE_PERMUTATION): New.
	(SLP_TREE_CODE): Likewise.
	(vect_stmt_dominates_stmt_p): Declare.
	* tree-vectorizer.c (vect_stmt_dominates_stmt_p): New function.
	* tree-vect-stmts.c (vect_model_simple_cost): Remove
	SLP_TREE_TWO_OPERATORS handling.
	* tree-vect-slp.c (_slp_tree::_slp_tree): Amend.
	(_slp_tree::~_slp_tree): Likewise.
	(vect_two_operations_perm_ok_p): Remove.
	(vect_build_slp_tree_1): Remove verification of two-operator
	permutation here.
	(vect_build_slp_tree_2): When we have two different operators
	build two computation SLP nodes and a blend.
	(vect_print_slp_tree): Print the lane permutation if it exists.
	(slp_copy_subtree): Copy it.
	(vect_slp_rearrange_stmts): Re-arrange it.
	(vect_slp_analyze_node_operations_1): Handle SLP_TREE_CODE
	VEC_PERM_EXPR explicitely.
	(vect_schedule_slp_instance): Likewise.  Remove old
	SLP_TREE_TWO_OPERATORS code.
	(vectorizable_slp_permutation): New function.