[llvm] [VP][RISCV] Add vp.cttz.elts intrinsic and its RISC-V codegen (PR #90502)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

I wish I could test the expansion logics (i.e. fallback) for this intrinsics but `vfirst` is in every RVV extensions and AArch64 doesn't really support the combination of VP intrinsics + scalable vectors.

https://github.com/llvm/llvm-project/pull/90502