Refactor x86 vectorized gather path

Refactor x86 vectorized gather path

The following moves the builtin decl gather vectorization path along
the internal function and emulated gather vectorization paths,
simplifying the existing function down to generating the call and
required conversions to the actual argument types.  This thereby
exposes the unique support of two times larger number of offset
or data vector lanes.  It also makes the code path handle SLP
in principle (but SLP build needs adjustments for this, patch coming).

	* tree-vect-stmts.cc (vect_build_gather_load_calls): Rename
	to ...
	(vect_build_one_gather_load_call): ... this.  Refactor,
	inline widening/narrowing support ...
	(vectorizable_load): ... here, do gather vectorization
	with builtin decls along other gather vectorization.