tree-optimization/95839 - teach SLP vectorization about vector inputs

This teaches SLP analysis about vector typed externals that are
fed into the SLP operations via lane extracting BIT_FIELD_REFs.
It shows that there's currently no good representation for
vector code on the SLP side so I went a half way and represent
such vector externals uses always using a SLP permutation node
with a single external SLP child which has a non-standard
representation of no scalar defs but only a vector def.  That
works best for shielding the rest of the vectorizer from it.

2020-06-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95839
	* tree-vect-slp.c (vect_slp_tree_uniform_p): Pre-existing
	vectors are not uniform.
	(vect_build_slp_tree_1): Handle BIT_FIELD_REFs of
	vector registers.
	(vect_build_slp_tree_2): For groups of lane extracts
	from a vector register generate a permute node
	with a special child representing the pre-existing vector.
	(vect_prologue_cost_for_slp): Pre-existing vectors cost nothing.
	(vect_slp_analyze_node_operations): Use SLP_TREE_LANES.
	(vectorizable_slp_permutation): Do not generate or cost identity
	permutes.
	(vect_schedule_slp_instance): Handle pre-existing vector
	that are function arguments.

	* gcc.dg/vect/bb-slp-pr95839-2.c: New testcase.

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