tree-optimization/114413 - SLP CSE after permute optimization

We currently fail to re-CSE SLP nodes after optimizing permutes
which results in off cost estimates.  For gcc.dg/vect/bb-slp-32.c
this shows in not re-using the SLP node with the load and arithmetic
for both the store and the reduction.  The following implements
CSE by re-bst-mapping nodes as finalization part of vect_optimize_slp.

I've tried to make the CSE part of permute materialization but it
isn't a very good fit there.  I've not bothered to implement something
more complete, also handling external defs or defs without
SLP_TREE_SCALAR_STMTS.

I realize this might result in more BB SLP which in turn might slow
down code given costing for BB SLP is difficult (even that we now
vectorize gcc.dg/vect/bb-slp-32.c on x86_64 might be not a good idea).
This is nevertheless feeding more accurate info to costing which is
good.

	PR tree-optimization/114413
	* tree-vect-slp.cc (release_scalar_stmts_to_slp_tree_map):
	New function, split out from ...
	(vect_analyze_slp): ... here.  Call it.
	(vect_cse_slp_nodes): New function.
	(vect_optimize_slp): Call it.

	* gcc.dg/vect/bb-slp-32.c: Expect CSE and vectorization on x86.

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