Code generate externals/invariants during the SLP graph walk

Code generate externals/invariants during the SLP graph walk

This generates vector defs for externals and invariants during the SLP
walk rather than as part of getting vectorized defs when vectorizing
the users.  This is a requirement to make sharing of external/invariant
nodes be reflected in actual code generation.

This temporarily adds a SLP_TREE_VEC_DEFS vector alongside the
SLP_TREE_VEC_STMTS one.  Eventually the latter can go away.

2020-05-27  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (_slp_tree::vec_defs): Add.
	(SLP_TREE_VEC_DEFS): Likewise.
	* tree-vect-slp.c (_slp_tree::_slp_tree): Adjust.
	(_slp_tree::~_slp_tree): Likewise.
	(vect_mask_constant_operand_p): Remove unused function.
	(vect_get_constant_vectors): Rename to...
	(vect_create_constant_vectors): ... this.  Take the
	invariant node as argument and code generate it.  Remove
	dead code, remove temporary asserts.  Pass a NULL stmt_info
	to vect_init_vector.
	(vect_get_slp_defs): Simplify.
	(vect_schedule_slp_instance): Code-generate externals and
	invariants using vect_create_constant_vectors.