We were computing &LOOP_VINFO_MASKS even for bb vectorisation,
which is UB.

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

gcc/
	PR tree-optimization/84634
	* tree-vect-stmts.c (vectorizable_store, vectorizable_load): Replace
	masks and masked_loop_p with a single loop_masks, making sure it's
	null for bb vectorization.

From-SVN: r258131