Allow SLP store of mixed external and constant

Allow SLP store of mixed external and constant

vect_build_slp_tree_1 rejected this during SLP discovery because it
ran into the rhs code comparison code for stores.  The following
skips that completely for loads and stores as those are handled
later anyway.

This needs a heuristic adjustment in vect_get_and_check_slp_defs
to avoid fallout with regard to BB vectorization and splitting
of a store group vs. demoting one operand to external.

gcc.dg/Wstringop-overflow-47.c needs adjustment given we now have
vast improvements for code generation.  gcc.dg/strlenopt-32.c
needs adjustment because the strlen pass doesn't handle

  _11 = {0, b_6(D)};
  __builtin_memcpy (&a, "foo.bar", 8);
  MEM <vector(2) char> [(char *)&a + 3B] = _11;
  _9 = strlen (&a);

I have opened PR117057 for this.

	* tree-vect-slp.cc (vect_build_slp_tree_1): Do not compare
	RHS codes for loads or stores.
	(vect_get_and_check_slp_defs): Only demote operand to external
	in case there is more than one operand.

	* gcc.dg/vect/slp-57.c: New testcase.
	* gcc.dg/Wstringop-overflow-47.c: Adjust.
	* gcc.dg/strlenopt-32.c: XFAIL parts.