VECT: Support SLP for MASK_LEN_GATHER_LOAD with dummy mask

VECT: Support SLP for MASK_LEN_GATHER_LOAD with dummy mask

This patch fixes following FAILs for RVV:
FAIL: gcc.dg/vect/vect-gather-1.c -flto -ffat-lto-objects  scan-tree-dump vect "Loop contains only SLP stmts"
FAIL: gcc.dg/vect/vect-gather-1.c scan-tree-dump vect "Loop contains only SLP stmts"

Bootstrap on X86 and regtest passed.

Ok for trunk ?

        PR tree-optimization/111721

gcc/ChangeLog:

	* tree-vect-slp.cc (vect_get_and_check_slp_defs): Support SLP for dummy mask -1.
	* tree-vect-stmts.cc (vectorizable_load): Ditto.