[RISCV] Improve splatPartsI64WithVL for fixed vector constants where Hi and Lo...

[RISCV] Improve splatPartsI64WithVL for fixed vector constants where Hi and Lo are the same and the VL is constant.

If doubling the VL will fit in a vsetivli, use it. It will be cheap
to change and cheap to change back.

This improves codegen from D158896.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D158896