[RISCV] Move some vector tests to rvv directory. NFC

I personally find it handy to be able to run the check-llvm-codegen-riscv-rvv target to test anything that involves vector codegen, so this patch moves some more vector-based test cases into that directory to increase its coverage.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147644