vect: Support length-based partial vectors approach

vect: Support length-based partial vectors approach

Power9 supports vector load/store instruction lxvl/stxvl which allow
us to operate partial vectors with one specific length.  This patch
extends some of current mask-based partial vectors support code for
length-based approach, also adds some length specific support code.
So far it assumes that we can only have one partial vectors approach
at the same time, it will disable to use partial vectors if both
approaches co-exist.

Like the description of optab len_load/len_store, the length-based
approach can have two flavors, one is length in bytes, the other is
length in lanes.  This patch is mainly implemented and tested for
length in bytes, but as Richard S. suggested, most of code has
considered both flavors.

This also introduces one parameter vect-partial-vector-usage allow
users to control when the loop vectorizer considers using partial
vectors as an alternative to falling back to scalar code.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_option_override_internal):
	Set param_vect_partial_vector_usage to 0 explicitly.
	* doc/invoke.texi (vect-partial-vector-usage): Document new option.
	* optabs-query.c (get_len_load_store_mode): New function.
	* optabs-query.h (get_len_load_store_mode): New declare.
	* params.opt (vect-partial-vector-usage): New.
	* tree-vect-loop-manip.c (vect_set_loop_controls_directly): Add the
	handlings for vectorization using length-based partial vectors, call
	vect_gen_len for length generation, and rename some variables with
	items instead of scalars.
	(vect_set_loop_condition_partial_vectors): Add the handlings for
	vectorization using length-based partial vectors.
	(vect_do_peeling): Allow remaining eiters less than epilogue vf for
	LOOP_VINFO_USING_PARTIAL_VECTORS_P.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Init
	epil_using_partial_vectors_p.
	(_loop_vec_info::~_loop_vec_info): Call release_vec_loop_controls
	for lengths destruction.
	(vect_verify_loop_lens): New function.
	(vect_analyze_loop): Add handlings for epilogue of loop when it's
	marked to use vectorization using partial vectors.
	(vect_analyze_loop_2): Add the check to allow only one vectorization
	approach using partial vectorization at the same time.  Check param
	vect-partial-vector-usage for partial vectors decision.  Mark
	LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P if the epilogue is
	considerable to use partial vectors.  Call release_vec_loop_controls
	for lengths destruction.
	(vect_estimate_min_profitable_iters): Adjust for loop vectorization
	using length-based partial vectors.
	(vect_record_loop_mask): Init factor to 1 for vectorization using
	mask-based partial vectors.
	(vect_record_loop_len): New function.
	(vect_get_loop_len): Likewise.
	* tree-vect-stmts.c (check_load_store_for_partial_vectors): Add
	checks for vectorization using length-based partial vectors.  Factor
	some code to lambda function get_valid_nvectors.
	(vectorizable_store): Add handlings when using length-based partial
	vectors.
	(vectorizable_load): Likewise.
	(vect_gen_len): New function.
	* tree-vectorizer.h (struct rgroup_controls): Add field factor
	mainly for length-based partial vectors.
	(vec_loop_lens): New typedef.
	(_loop_vec_info): Add lens and epil_using_partial_vectors_p.
	(LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P): New macro.
	(LOOP_VINFO_LENS): Likewise.
	(LOOP_VINFO_FULLY_WITH_LENGTH_P): Likewise.
	(vect_record_loop_len): New declare.
	(vect_get_loop_len): Likewise.
	(vect_gen_len): Likewise.