https://github.com/llvm/llvm-project/pull/153927 incorrectly cast using
a hardcoded reduction factor of two, rather than using the parameter.
This caused false negatives but not false positives. (The only incorrect
case was a reduction factor of four; if four values {A,B,C,D} are being
reduced, the result is fully zero iff {A,B} and {C,D} are both zero
after pairwise reduction. If only one of those reduced pairs is zero,
then the quadwise reduction is non-zero.)
1cc84bcc08