[clang] [libc] [Clang] Add width handling for <gpuintrin.h> shuffle helper (PR #125896)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> test?

I could add one in `libc`, otherwise I don't see great value in it. This is directly lifted from existing implementations so I'm not too concerned about its correctness and I did a sanity check on it.

https://github.com/llvm/llvm-project/pull/125896