[llvm] [RISCV] Use vsetivli instead of `x0,x0` form to retain SEW/LMUL when AVL is imm (PR #169307)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

Closed as this may not be beneficial. I will investigate these cases where we can reduce more vtype toggles and try to make it general.

https://github.com/llvm/llvm-project/pull/169307