Use loop->safelen rather than loop->force_vectorize

Use loop->safelen rather than loop->force_vectorize

...since the latter doesn't guarantee independence by itself.

2018-03-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-data-refs.c (vect_analyze_data_ref_dependence)
	(vect_analyze_data_ref_access): Use loop->safe_len rather than
	loop->force_vectorize to check whether there is no alias.

gcc/testsuite/
	* gcc.dg/vect/vect-alias-check-13.c: New test.

From-SVN: r258130