Split mask checking out of vectorizable_mask_load_store

Split mask checking out of vectorizable_mask_load_store

This patch splits the mask argument checking out of
vectorizable_mask_load_store, so that a later patch can use it in both
vectorizable_load and vectorizable_store.  It also adds dump messages
for false returns.  This is mostly useful for the TYPE_VECTOR_SUBPARTS
check, which can fail if pattern recognition didn't convert the mask
properly.

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

gcc/
	* tree-vect-stmts.c (vect_check_load_store_mask): New function,
	split out from...
	(vectorizable_mask_load_store): ...here.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256212 138bc75d-0d04-0410-961f-82ee72b054a4