IFN/optabs: Support vector load/store with length

IFN/optabs: Support vector load/store with length

This patch is to add the internal function and optabs support for
vector load/store with length.

For the vector load/store with length optab, the length item would
be measured in lanes by default.  For the targets which support
length measured in bytes like Power, they should only define VnQI
modes to wrap the other same size vector modes.  If the length is
larger than total lane/byte count of the given mode, the behavior
is undefined.  For the remaining lanes/bytes which isn't specified
by length, they would be taken as undefined value.

gcc/ChangeLog:

	* doc/md.texi (len_load_@var{m}): Document.
	(len_store_@var{m}): Likewise.
	* internal-fn.c (len_load_direct): New macro.
	(len_store_direct): Likewise.
	(expand_len_load_optab_fn): Likewise.
	(expand_len_store_optab_fn): Likewise.
	(direct_len_load_optab_supported_p): Likewise.
	(direct_len_store_optab_supported_p): Likewise.
	(expand_mask_load_optab_fn): New macro.  Original renamed to ...
	(expand_partial_load_optab_fn): ... here.  Add handlings for
	len_load_optab.
	(expand_mask_store_optab_fn): New macro.  Original renamed to ...
	(expand_partial_store_optab_fn): ... here. Add handlings for
	len_store_optab.
	(internal_load_fn_p): Handle IFN_LEN_LOAD.
	(internal_store_fn_p): Handle IFN_LEN_STORE.
	(internal_fn_stored_value_index): Handle IFN_LEN_STORE.
	* internal-fn.def (LEN_LOAD): New internal function.
	(LEN_STORE): Likewise.
	* optabs.def (len_load_optab, len_store_optab): New optab.