[LV][RISCV] Regenerate reduction tests with UTC. NFC

Previously these tests used optimization remarks to check the chosen VF,
but I don't think thats needed if we can see from the types used in the
vectorized body. We can just use UTC to generate the body instead.

Whilst we're here, also simplify the opt invocation to remove unneeded
options and rename from scalable-reductions.ll -> reductions.ll, seeing
as it's not specifically testing for scalable VFs anymore.

Author: lukel97