vec: don't select partial vectors when unnecessary

vec: don't select partial vectors when unnecessary

gcc/ChangeLog

2020-09-09  Andrea Corallo  <andrea.corallo@arm.com>

	* tree-vect-loop.c (vect_need_peeling_or_partial_vectors_p): New
	function.
	(vect_analyze_loop_2): Make use of it not to select partial
	vectors if no peel is required.
	(determine_peel_for_niter): Move out some logic into
	'vect_need_peeling_or_partial_vectors_p'.

gcc/testsuite/ChangeLog

2020-09-09  Andrea Corallo  <andrea.corallo@arm.com>

	* gcc.target/aarch64/sve/cost_model_10.c: New test.
	* gcc.target/aarch64/sve/clastb_8.c: Update test for new
	vectorization strategy.
	* gcc.target/aarch64/sve/cost_model_5.c: Likewise.
	* gcc.target/aarch64/sve/struct_vect_14.c: Likewise.
	* gcc.target/aarch64/sve/struct_vect_15.c: Likewise.
	* gcc.target/aarch64/sve/struct_vect_16.c: Likewise.
	* gcc.target/aarch64/sve/struct_vect_17.c: Likewise.