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

This adds tests for fixed and scalable vectors where we have a binary op
on two splats that could be scalarized. Normally this would be
scalarized in the middle-end by VectorCombine, but as noted in
https://reviews.llvm.org/D159190, this pattern can crop up during
CodeGen afterwards.

Note that a combine already exists for this, but on RISC-V currently it
only works on scalable vectors where the element type == XLEN. See
llvm#65068 and llvm#65072

Author: lukel97