This patch adds a bunch of flags to dr_with_seg_len_pair_t,
for use by later patches.  The update to tree-loop-distribution.c
is conservatively correct, but might be tweakable later.

2019-11-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-data-ref.h (DR_ALIAS_RAW, DR_ALIAS_WAR, DR_ALIAS_WAW)
	(DR_ALIAS_ARBITRARY, DR_ALIAS_SWAPPED, DR_ALIAS_UNSWAPPED): New flags.
	(dr_with_seg_len_pair_t::sequencing): New enum.
	(dr_with_seg_len_pair_t::flags): New member variable.
	(dr_with_seg_len_pair_t::dr_with_seg_len_pair_t): Take a sequencing
	parameter and initialize the flags member variable.
	* tree-loop-distribution.c (compute_alias_check_pairs): Update
	call accordingly.
	* tree-vect-data-refs.c (vect_prune_runtime_alias_test_list): Likewise.
	Ensure the two data references in an alias pair are in statement
	order, if there is a defined order.
	* tree-data-ref.c (prune_runtime_alias_test_list): Use
	DR_ALIAS_SWAPPED and DR_ALIAS_UNSWAPPED to record whether we've
	swapped the references in a dr_with_seg_len_pair_t.  OR together
	the flags when merging two dr_with_seg_len_pair_ts.  After merging,
	try to restore the original dr_with_seg_len order, updating the
	flags if that fails.

From-SVN: r278350