Allow scalar fallback for pattern root stmt

Allow scalar fallback for pattern root stmt

This adjusts the SLP build to allow a pattern root stmt to be
built from scalars.  I've noticed this in PR98211 where we fail
to promote a SLP subtree to a simple splat operation and instead
emit a series of uniform vector operations.  The bb-slp-div-1.c
testcase is now vectorized on x86_64 but only the store so I
adjusted it to expect the load to be vectorized.

2020-12-10  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_get_and_check_slp_defs): Do
	not mark the defs to occur in a pattern if it is the
	pattern root and record the original stmt defs in that
	case.

	* gcc.dg/vect/bb-slp-div-1.c: Expect the load to be
	vectorized.