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

[Wang Pengcheng via cfe-commits]

wangpc-pp wrote:

> Also I guess most of RVV intrinsic could add `const` too, that could help some generic optimization work better like CSE.

This PR doesn't add the `const` attribute, I don't know if adding `const` may help to optimize.

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