[llvm] Perform bitreverse using AVX512 GFNI for i32 and i64. (PR #81764)

[via llvm-commits]

shamithoke wrote:

> @shamithoke I forgot to ask - why didn't you extend this for i8/i16 cases?

I needed the bitreverse for i32/i64 on AVX, so I just handled those case. 
Let me make a PR to add i8 and i16. 

https://github.com/llvm/llvm-project/pull/81764