[RISCV] Add tests where bin ops of splats could be scalarized. NFC

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 currently it only works on
scalable vectors where the element type == XLEN. See #65068 and #65072

Author: lukel97