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

[Kito Cheng via cfe-commits]

kito-cheng wrote:

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

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