tree-optimization/116352 - SLP scheduling and stmt order

The PR uncovers unchecked constraints on the ability to code-generate
with SLP but also latent issues with regard to stmt order checking
since loop (early-break) and BB (for quite some time) vectorization
are no longer constraint to single-BBs.  In particular get_later_stmt
simply compares UIDs of stmts, but that's only reliable when they
are in the same BB.

For the PR in question the problematical case is demoting a SLP node
to external which fails to check we can actually code generate this
in the way we do (using get_later_stmt).  The following thus adds
checking that we demote to external only when all defs are from
the same BB.

We no longer vectorize gcc.dg/vect/bb-slp-49.c but the testcase was
for a wrong-code issue and the vectorization done is a no-op.

	PR tree-optimization/116352
	PR tree-optimization/117876
	* tree-vect-slp.cc (vect_slp_can_convert_to_external): New.
	(vect_slp_convert_to_external): Call it.
	(vect_build_slp_tree_2): Likewise.

	* gcc.dg/vect/pr116352.c: New testcase.
	* gcc.dg/vect/bb-slp-49.c: Remove vectorization check.

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