ifn: Add else-operand handling.

This patch adds else-operand handling to the internal functions.

gcc/ChangeLog:

	* internal-fn.cc (add_mask_and_len_args): Rename...
	(add_mask_else_and_len_args): ...to this and add else handling.
	(expand_partial_load_optab_fn): Use adjusted function.
	(expand_partial_store_optab_fn): Ditto.
	(expand_scatter_store_optab_fn): Ditto.
	(expand_gather_load_optab_fn): Ditto.
	(internal_fn_len_index): Add else handling.
	(internal_fn_else_index): Ditto.
	(internal_fn_mask_index): Ditto.
	(get_supported_else_vals): New function.
	(supported_else_val_p): New function.
	(internal_gather_scatter_fn_supported_p): Add else operand.
	* internal-fn.h (internal_gather_scatter_fn_supported_p): Define
	else constants.
	(MASK_LOAD_ELSE_ZERO): Ditto.
	(MASK_LOAD_ELSE_M1): Ditto.
	(MASK_LOAD_ELSE_UNDEFINED): Ditto.
	(get_supported_else_vals): Declare.
	(supported_else_val_p): Ditto.