enfoce SLP_TREE_VECTYPE for invariants

This tries to enforce a set SLP_TREE_VECTYPE in vect_get_constant_vectors
and provides some infrastructure for setting it in the vectorizable_*
functions, amending those.

2020-05-22  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (vect_is_simple_use): New overload.
	(vect_maybe_update_slp_op_vectype): New.
	* tree-vect-stmts.c (vect_is_simple_use): New overload
	accessing operands of SLP vs. non-SLP operation transparently.
	(vect_maybe_update_slp_op_vectype): New function updating
	the possibly shared SLP operands vector type.
	(vectorizable_operation): Be a bit more SLP vs non-SLP agnostic
	using the new vect_is_simple_use overload;  update SLP invariant
	operand nodes vector type.
	(vectorizable_comparison): Likewise.
	(vectorizable_call): Likewise.
	(vectorizable_conversion): Likewise.
	(vectorizable_shift): Likewise.
	(vectorizable_store): Likewise.
	(vectorizable_condition): Likewise.
	(vectorizable_assignment): Likewise.
	* tree-vect-loop.c (vectorizable_reduction): Likewise.
	* tree-vect-slp.c (vect_get_constant_vectors): Enforce
	present SLP_TREE_VECTYPE and check it matches previous
	behavior.