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

[Luke Lau via llvm-commits]

lukel97 wrote:

> I want to echo @topperc’s concern that if VL is zero for a div then we should not scalarize since that would lead to division by zero in scalar land, but in vector land no division occurs.
> 
> Can we explicitly test this case?
> 
> EDIT: Sorry, we may not need to do this because I think VL is never zero when using vectors and not vp intrinsics ?

Yeah, with regular vector arithmetic there's no concept of the EVL, I double checked anyway and vp nodes don't get run through this combine

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