[clang] [Clang][RISCV] Add assumptions to vsetvli/vsetvlimax (PR #79975)

[Wang Pengcheng via cfe-commits]

wangpc-pp wrote:

> Can we implement this in `computeKnownBitsFromOperator/getRangeForIntrinsic`?
> 
> https://github.com/llvm/llvm-project/blob/b21e3282864c9f7ad656c64bc375f5869ef76d19/llvm/lib/Analysis/ValueTracking.cpp#L1578-L1584

Thanks! We can implement this partly (will create another PR to do this) as we can't represent some assumptions related to runtime value `avl` like `vl<avl` with `KnownBits` (IIUC).

https://github.com/llvm/llvm-project/pull/79975