vect: Move VMAT_GATHER_SCATTER handlings from final loop nest

Following Richi's suggestion [1], this patch is to move the
handlings on VMAT_GATHER_SCATTER in the final loop nest
of function vectorizable_load to its own loop.  Basically
it duplicates the final loop nest, clean up some useless
set up code for the case of VMAT_GATHER_SCATTER, remove some
unreachable code.  Also remove the corresponding handlings
in the final loop nest.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-June/623329.html

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_load): Move the handlings on
	VMAT_GATHER_SCATTER in the final loop nest to its own loop,
	and update the final nest accordingly.