middle-end: updated niters analysis to handle multiple exits.

This second part updates niters analysis to be able to analyze any number of
exits.  If we have multiple exits we determine the main exit by finding the
first counting IV.

The change allows the vectorizer to pass analysis for multiple loops, but we
later gracefully reject them.  It does however allow us to test if the exit
handling is using the right exit everywhere.

Additionally since we analyze all exits, we now return all conditions for them
and determine which condition belongs to the main exit.

The main condition is needed because the vectorizer needs to ignore the main IV
condition during vectorization as it will replace it during codegen.

To track versioned loops we extend the contract between ifcvt and the vectorizer
to store the exit number in aux so that we can match it up again during peeling.

gcc/ChangeLog:

	* tree-if-conv.cc (tree_if_conversion): Record exits in aux.
	* tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg): Use
	it.
	* tree-vect-loop.cc (vect_get_loop_niters): Determine main exit.
	(vec_init_loop_exit_info): Extend analysis when multiple exits.
	(vect_analyze_loop_form): Record conds and determine main cond.
	(vect_create_loop_vinfo): Extend bookkeeping of conds.
	(vect_analyze_loop): Release conds.
	* tree-vectorizer.h (LOOP_VINFO_LOOP_CONDS,
	LOOP_VINFO_LOOP_IV_COND):  New.
	(struct vect_loop_form_info): Add conds, alt_loop_conds;
	(struct loop_vec_info): Add conds, loop_iv_cond.

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