[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:

Besides the potential uarch impact, this change removes more vtype toggles. For in-tree tests: `120 files changed, 2518 insertions(+), 2721 deletions(-)`.

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