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

lukel97 merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

lukel97:scalarize-binop-of-splats-build-vector/testslukel97/llvm-project:scalarize-binop-of-splats-build-vector/testsCopy head branch name to clipboard