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

[Krste Asanovic via llvm-commits]

kasanovic wrote:

The repeated writes to vl required by the vsetivli form will likely be detrimental to OoO microarchitectures that rename vl, so should not be the default when we can use the other form that reuses existing vl value.

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