tree-optimization/104445 - check for vector extraction support

This adds a missing check to epilogue reduction re-use, namely
that we can do hi/lo extracts from the vector when demoting it
to the epilogue vector size.

I've chosen to add a can_vec_extract helper to optabs-query.h,
in the future we might want to simplify the vectorizers life by
handling vector-from-vector extraction via BIT_FIELD_REFs during
RTL expansion via the mode punning when the vec_extract is not
directly supported.

I'm not 100% sure we can always do the punning of the
vec_extract result to a vector mode of the same size, but then
I'm also not sure how to check for that (the vectorizer doesn't
in other places it does that at the moment, but I suppose we
eventually just go through memory there)?

2022-02-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104445
	PR tree-optimization/102832
	* optabs-query.h (can_vec_extract): New.
	* optabs-query.cc (can_vec_extract): Likewise.
	* tree-vect-loop.cc (vect_find_reusable_accumulator): Check
	we can extract a hi/lo part from the larger vector, rework
	check iteration from larger to smaller sizes.

	* gcc.dg/vect/pr104445.c: New testcase.

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