vect: Support masked gather loads with SLP

vect: Support masked gather loads with SLP

This patch extends the previous SLP gather load support so
that it can handle masked loads too.

gcc/
	* tree-vect-slp.c (arg1_arg4_map): New variable.
	(vect_get_operand_map): Handle IFN_MASK_GATHER_LOAD.
	(vect_build_slp_tree_1): Likewise.
	(vect_build_slp_tree_2): Likewise.
	* tree-vect-stmts.c (vectorizable_load): Expect the mask to be
	the last SLP child node rather than the first.

gcc/testsuite/
	* gcc.dg/vect/vect-gather-3.c: New test.
	* gcc.dg/vect/vect-gather-4.c: Likewise.
	* gcc.target/aarch64/sve/mask_gather_load_8.c: Likewise.