[clang] [Clang][RISCV] Add assumptions for vsetvli and vsetvlimax results (PR #218311)

[Pengcheng Wang via cfe-commits]

wangpc-pp wrote:

Good point! I also did a `instCombineIntrinsic` implementation: https://github.com/llvm/llvm-project/pull/218652.

But yes, I don't think we need to change Clang. We'd better do it in middle-end so other frontends like Rust/MLIR may also benefit from it too. :-)

https://github.com/llvm/llvm-project/pull/218311