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

[Maciej Gabka via cfe-commits]

mgabka wrote:

Added a message here: https://discourse.llvm.org/t/rfc-promoting-experimental-interleave2-deinterleave2-reverse-splice-and-stepvector-intrinsics-to-first-class-intrinsics/78414

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