[clang] Fix implicit truncation of `select` non-bool vector conditions (PR #166279)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

> Looks good to me. Should get a review from someone who knows more about long vectors though, and determine if/how we should come up with a general solution that would work with long vectors.

Not sure we have to worry about this yet. Much more than just select will need to be updated for long vectors. I think we can punt on that for now.

https://github.com/llvm/llvm-project/pull/166279