Split vector loop analysis into main and epilogue analysis

Split vector loop analysis into main and epilogue analysis

As discussed this splits the analysis loop into two, first settling
on a vector mode used for the main loop and only then analyzing
the epilogue of that for possible vectorization.  That makes it
easier to put in support for unrolled main loops.

On the way I've realized some cleanup opportunities, namely caching
n_stmts in vec_info_shared (it's computed by dataref analysis)
avoiding to pass that around and setting/clearing loop->aux
during analysis - try_vectorize_loop_1 will ultimatively set it
on those we vectorize.

This also gets rid of the previously introduced callback in
vect_analyze_loop_1 in favor of making that advance the mode iterator.
I'm now pushing VOIDmode explicitely into the vector_modes array
which makes the re-start on the epilogue side a bit more
straight-forward.  Note that will now use auto-detection of the
vector mode in case the main loop used it and we want to try
LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P and the first mode from
the target array if not.  I've added a comment that says we may
want to make sure we don't try vectorizing the epilogue with a
bigger vector size than the main loop but the situation isn't
very likely to appear in practice I guess (and it was also present
before this change).

In principle this change should not change vectorization decisions
but the way we handled re-analyzing epilogues as main loops makes
me only 99% sure that it does.

2021-11-05  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (vec_info_shared::n_stmts): Add.
	(LOOP_VINFO_N_STMTS): Likewise.
	(vec_info_for_bb): Remove unused function.
	* tree-vectorizer.c (vec_info_shared::vec_info_shared):
	Initialize n_stmts member.
	* tree-vect-loop.c: Remove INCLUDE_FUNCTIONAL.
	(vect_create_loop_vinfo): Do not set loop->aux.
	(vect_analyze_loop_2): Do not get n_stmts as argument,
	instead use LOOP_VINFO_N_STMTS.  Set LOOP_VINFO_VECTORIZABLE_P
	here.
	(vect_analyze_loop_1): Remove callback, get the mode iterator
	and autodetected_vector_mode as argument, advancing the
	iterator and initializing autodetected_vector_mode here.
	(vect_analyze_loop): Split analysis loop into two, first
	processing main loops only and then epilogues.