[clang] [llvm] Move several vector intrinsics out of experimental namespace (PR #88748)

[Cullen Rhodes via cfe-commits]

c-rhodes wrote:

Thanks for patch. We use `interleave2` in MLIR so it'll need updating there as well.

I don't know what the policy is for promoting intrinsics from experimental to first-class or if it's documented anywhere (?), but I would expect this to be accompanied with an RFC / announcement on Discourse.

Also, what about `stepvector`?

https://github.com/llvm/llvm-project/pull/88748