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

[Craig Topper via llvm-commits]

topperc wrote:

vsetvli x0, x0 can't change vl. It can only change vtype and set vill. If the decoder was able to retrieve the vl from a vsetvli, it has to remember it for each subsequent vector instruction. Wouldn't it also remember it across the the vsetvli x0, x0?

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