[LoopVectorize] Test in-loop reductions with tail folding for scalable vectors

Adds `-prefer-inloop-reductions` to the RUN line of sve-tail-folding.ll & adds
a new test where in-loop reductions cannot be used (`@cond_xor_reduction`). NFC.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D117578