Add a vect_masked_store target selector

Add a vect_masked_store target selector

This patch adds a target selector that says whether the target
supports IFN_MASK_STORE.

2017-11-09  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* doc/sourcebuild.texi (vect_masked_store): Document.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_vect_masked_store):
	New proc.
	* gcc.dg/vect/vect-cselim-1.c (foo): Mention that the second loop
	is vectorizable with masked stores.  Update scan-tree-dump-times
	accordingly.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254597