Make vectorizable_load/store handle IFN_MASK_LOAD/STORE

Make vectorizable_load/store handle IFN_MASK_LOAD/STORE

After the previous patches, it's easier to see that the remaining
inlined transform code in vectorizable_mask_load_store is just a
cut-down version of the VMAT_CONTIGUOUS handling in vectorizable_load
and vectorizable_store.  This patch therefore makes those functions
handle masked loads and stores instead.

This makes it easier to handle more forms of masked load and store
without duplicating logic from the unmasked forms.  It also helps with
support for fully-masked loops.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-stmts.c (vect_get_store_rhs): New function.
	(vectorizable_mask_load_store): Delete.
	(vectorizable_call): Return false for masked loads and stores.
	(vectorizable_store): Handle IFN_MASK_STORE.  Use vect_get_store_rhs
	instead of gimple_assign_rhs1.
	(vectorizable_load): Handle IFN_MASK_LOAD.
	(vect_transform_stmt): Don't set is_store for call_vec_info_type.

From-SVN: r256216