tree-optimization/111115 - SLP of masked stores

tree-optimization/111115 - SLP of masked stores

The following adds the capability to do SLP on .MASK_STORE, I do not
plan to add interleaving support.

	PR tree-optimization/111115
gcc/
	* tree-vectorizer.h (vect_slp_child_index_for_operand): New.
	* tree-vect-data-refs.cc (can_group_stmts_p): Also group
	.MASK_STORE.
	* tree-vect-slp.cc (arg3_arg2_map): New.
	(vect_get_operand_map): Handle IFN_MASK_STORE.
	(vect_slp_child_index_for_operand): New function.
	(vect_build_slp_tree_1): Handle statements with no LHS,
	masked store ifns.
	(vect_remove_slp_scalar_calls): Likewise.
	* tree-vect-stmts.cc (vect_check_store_rhs): Lookup the
	SLP child corresponding to the ifn value index.
	(vectorizable_store): Likewise for the mask index.  Support
	masked stores.
	(vectorizable_load): Lookup the SLP child corresponding to the
	ifn mask index.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_vect_masked_store):
	Supported with check_avx_available.
	* gcc.dg/vect/slp-mask-store-1.c: New testcase.