[llvm] [RISCV] Prefer VLS over VLA if costs are equal (PR #100564)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> Worth noting is that adding -mrvv-vector-bits=zvl to both sides results in exactly the same code, and that code appears reasonable.

I think it is because we are using fixed-vectors with `-mrvv-vector-bits=zvl`.

https://github.com/llvm/llvm-project/pull/100564