vect: Use LOOP_VINFO_DATAREFS and LOOP_VINFO_DDRS consistently

vect: Use LOOP_VINFO_DATAREFS and LOOP_VINFO_DDRS consistently

Minor code refactorings in tree-vect-data-refs.c and tree-vect-loop.c.
Use LOOP_VINFO_DATAREFS and LOOP_VINFO_DDRS when possible and rename
several parameters to make code more consistent.

2020-06-13  Felix Yang  <felix.yang@huawei.com>

gcc/
	* tree-vect-data-refs.c (vect_verify_datarefs_alignment): Rename
	parameter to loop_vinfo and update uses.  Use LOOP_VINFO_DATAREFS
	when possible.
	(vect_analyze_data_refs_alignment): Likewise, and use LOOP_VINFO_DDRS
	when possible.
	* tree-vect-loop.c (vect_dissolve_slp_only_groups): Use
	LOOP_VINFO_DATAREFS when possible.
	(update_epilogue_loop_vinfo): Likewise.