Hi,
As commented in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80815#c1,
We can relax minimal segment length of DR_B for merging.  With this change,
the new test can be improved to only one alias check.  Note the
condition is still accurate after this patch, it won't introduce false
alias.
Bootstrap and test on x86_64 and AArch64, is it OK?

2017-05-22  Bin Cheng  <bin.cheng@arm.com>

	* tree-data-ref.c (prune_runtime_alias_test_list): Relax minimal
	segment length for dr_b.

gcc/testsuite/ChangeLog
2017-05-22  Bin Cheng  <bin.cheng@arm.com>

	* gcc.dg/vect/pr80815-3.c: New test.