Split gather load handling out of vectorizable_{mask_load_store,load}

Split gather load handling out of vectorizable_{mask_load_store,load}

vectorizable_mask_load_store and vectorizable_load used the same
code to build a gather load call, except that the former also
vectorised a mask argument and used it for both the merge and mask
inputs.  The latter instead used a merge input of zero and a mask
input of all-ones.  This patch splits it out into a subroutine.

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

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

From-SVN: r256215