vect: Use vect_relevant_for_alignment_p consistently

vect: Use vect_relevant_for_alignment_p consistently

Minor code refactorings in tree-vect-data-refs.c.  When trying loop peeling
to force alignment, call vect_relevant_for_alignment_p to filter out data
references in the loop whose alignment is irrelevant.

2020-06-18  Felix Yang  <felix.yang@huawei.com>

gcc/
	* tree-vect-data-refs.c (vect_enhance_data_refs_alignment): Call
	vect_relevant_for_alignment_p to filter out data references in
	the loop whose alignment is irrelevant when trying loop peeling
	to force alignment.