Better recover from SLP reassociation fails during discovery

When we decide to not process a association chain of size two and
that would also mismatch with a different chain size on another lane
we shouldn't fail discovery hard at this point.  Instead let the
regular discovery figure out matching lanes so the parent can
decide to perform operand swapping or we can split groups at better
points rather than forcefully splitting away the first single lane.

For example on gcc.dg/vect/vect-strided-u8-i8.c we now see two
groups of size 4 feeding the store instead of groups of size 1,
three, two, one and one.

	* tree-vect-slp.cc (vect_build_slp_tree_2): On reassociation
	chain length mismatch do not fail discovery of the node
	but try without re-associating to compute a better matches[].
	Provide a reassociation failure hint in the dump.
	(vect_slp_analyze_node_operations): Avoid stray failure
	dumping.
	(vectorizable_slp_permutation_1): Dump the address of the
	SLP node representing the permutation.

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