Extended Description

Running opt -S -slp-vectorizer $TEST_FILE, the function gets SLP-vectorized; Running opt -S -reassociate -slp-vectorizer $TEST_FILE, the function doesn't get SLP-vectorized.

This is because the reassociation pass re-associates instructions like acc = add ith_element, acc to acc = add acc, ith_element, but didn't re-assocate the first instruction acc = add first_element, second_element to acc = add second_element, first_element.

However, as add is associative, a smarter SLP vectorizer could have ignored the associations and vectorize the unordered adds anyway.

I'm not sure where to put the fix on.