Ignore ignored operands in vect_get_and_check_slp_defs

Ignore ignored operands in vect_get_and_check_slp_defs

This passes down skip_args to vect_get_and_check_slp_defs to skip
ignored ops there, too and not fail SLP discovery.  This fixes
gcc.target/aarch64/sve/reduc_strict_5.c

2020-10-28  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_get_and_check_slp_defs): For skipped
	args just push NULLs and vect_uninitialized_def.
	(vect_build_slp_tree_2): Allocate skip_args for all ops
	and pass it down to vect_get_and_check_slp_defs.