[vect] Add main vectorized loop unrolling

[vect] Add main vectorized loop unrolling

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_estimate_min_profitable_iters): Pass new
	argument suggested_unroll_factor.
	(vect_analyze_loop_costing): Likewise.
	(_loop_vec_info::_loop_vec_info): Initialize new member
	suggested_unroll_factor.
	(vect_determine_partial_vectors_and_peeling): Make epilogue of unrolled
	main loop use partial vectors.
	(vect_analyze_loop_2): Pass and use new argument
	suggested_unroll_factor.
	(vect_analyze_loop_1): Change to intialize local
	suggested_unroll_factor and use it.
	(vectorizable_reduction): Don't use single_defuse_cycle when unrolling.
	* tree-vectorizer.h (_loop_vec_info::_loop_vec_info): Add new member
	suggested_unroll_factor.
	(vector_costs::vector_costs): Add new member m_suggested_unroll_factor.
	(vector_costs::suggested_unroll_factor): New getter function.
	(finish_cost): Set return argument suggested_unroll_factor.