Support -m[no-]gather -m[no-]scatter to enable/disable vectorization for all...

Support -m[no-]gather -m[no-]scatter to enable/disable vectorization for all gather/scatter instructions

Rename original use_gather to use_gather_8parts, Support
-mtune-ctrl={,^}use_gather to set/clear tune features
use_gather_{2parts, 4parts, 8parts}. Support the new option -mgather
as alias of -mtune-ctrl=, use_gather, ^use_gather.

Similar for use_scatter.

gcc/ChangeLog:

	* config/i386/i386-builtins.cc
	(ix86_vectorize_builtin_gather): Adjust for use_gather_8parts.
	* config/i386/i386-options.cc (parse_mtune_ctrl_str):
	Set/Clear tune features use_{gather,scatter}_{2parts, 4parts,
	8parts} for -mtune-crtl={,^}{use_gather,use_scatter}.
	* config/i386/i386.cc (ix86_vectorize_builtin_scatter): Adjust
	for use_scatter_8parts
	* config/i386/i386.h (TARGET_USE_GATHER): Rename to ..
	(TARGET_USE_GATHER_8PARTS): .. this.
	(TARGET_USE_SCATTER): Rename to ..
	(TARGET_USE_SCATTER_8PARTS): .. this.
	* config/i386/x86-tune.def (X86_TUNE_USE_GATHER): Rename to
	(X86_TUNE_USE_GATHER_8PARTS): .. this.
	(X86_TUNE_USE_SCATTER): Rename to
	(X86_TUNE_USE_SCATTER_8PARTS): .. this.
	* config/i386/i386.opt: Add new options mgather, mscatter.