vect: Add maskload else value support.

This patch adds an else operand to vectorized masked load calls.
The current implementation adds else-value arguments to the respective
target-querying functions that is used to supply the vectorizer with the
proper else value.

We query the target for its supported else operand and uses that for the
maskload call.  If necessary, i.e. if the mode has padding bits and if
the else operand is nonzero, a VEC_COND enforcing a zero else value is
emitted.

gcc/ChangeLog:

	* optabs-query.cc (supports_vec_convert_optab_p): Return icode.
	(get_supported_else_val): Return supported else value for
	optab's operand at index.
	(supports_vec_gather_load_p): Add else argument.
	(supports_vec_scatter_store_p): Ditto.
	* optabs-query.h (supports_vec_gather_load_p): Ditto.
	(get_supported_else_val): Ditto.
	* optabs-tree.cc (target_supports_mask_load_store_p): Ditto.
	(can_vec_mask_load_store_p): Ditto.
	(target_supports_len_load_store_p): Ditto.
	(get_len_load_store_mode): Ditto.
	* optabs-tree.h (target_supports_mask_load_store_p): Ditto.
	(can_vec_mask_load_store_p): Ditto.
	* tree-vect-data-refs.cc (vect_lanes_optab_supported_p): Ditto.
	(vect_gather_scatter_fn_p): Ditto.
	(vect_check_gather_scatter): Ditto.
	(vect_load_lanes_supported): Ditto.
	* tree-vect-patterns.cc (vect_recog_gather_scatter_pattern):
	Ditto.
	* tree-vect-slp.cc (vect_get_operand_map): Adjust indices for
	else operand.
	(vect_slp_analyze_node_operations): Skip undefined else operand.
	* tree-vect-stmts.cc (exist_non_indexing_operands_for_use_p):
	Add else operand handling.
	(vect_get_vec_defs_for_operand): Handle undefined else operand.
	(check_load_store_for_partial_vectors): Add else argument.
	(vect_truncate_gather_scatter_offset): Ditto.
	(vect_use_strided_gather_scatters_p): Ditto.
	(get_group_load_store_type): Ditto.
	(get_load_store_type): Ditto.
	(vect_get_mask_load_else): Ditto.
	(vect_get_else_val_from_tree): Ditto.
	(vect_build_one_gather_load_call): Add zero else operand.
	(vectorizable_load): Use else operand.
	* tree-vectorizer.h (vect_gather_scatter_fn_p): Add else
	argument.
	(vect_load_lanes_supported): Ditto.
	(vect_get_mask_load_else): Ditto.
	(vect_get_else_val_from_tree): Ditto.