Avoid unnecessary peeling for gaps with LD3

Avoid unnecessary peeling for gaps with LD3

vectorizable_load forces peeling for gaps if the vectorisation factor
is not a multiple of the group size, since in that case we'd normally load
beyond the original scalar accesses but drop the excess elements as part
of a following permute:

          if (loop_vinfo
              && ! STMT_VINFO_STRIDED_P (stmt_info)
              && (GROUP_GAP (vinfo_for_stmt (first_stmt)) != 0
                  || (!slp && vf % GROUP_SIZE (vinfo_for_stmt (first_stmt)) != 0)))

This isn't necessary for LOAD_LANES though, since it loads only the
data needed and does the permute itself.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/
	* tree-vect-stmts.c (vectorizable_load): Reorder checks so that
	load_lanes/grouped_load classification comes first.  Don't check
	whether the vectorization factor is a multiple of the group size
	for load_lanes.

gcc/testsuite/
	* gcc.dg/vect/vect-load-lanes-peeling-1.c: New test.

From-SVN: r236632