[llvm] [RISCV] Add tests where bin ops of splats could be scalarized. NFC (PR #65747)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

Will you try and scalarize instructions like div and rem which are more expensive and may need cost modeling? Will you scalarize f16 vectors in the case that there is no f16 scalar, which requires extra instructions to convert to f32 first?

If so, it could be nice to have tests for these instructions/types. 

https://github.com/llvm/llvm-project/pull/65747