[llvm] [RISCV] Support postRA vsetvl insertion pass (PR #70549)

[Piyou Chen via llvm-commits]

BeMg wrote:

Because #72950 has landed. I'm going to rebase the patch with the main branch. And make this patch focus on the postRA vsetvl insertion pass and its impact on lit test.

https://github.com/llvm/llvm-project/pull/70549