[WebAssembly] Prototype extending pairwise add instructions

As proposed in https://github.com/WebAssembly/simd/pull/380. This commit makes
the new instructions available only via clang builtins and LLVM intrinsics to
make their use opt-in while they are still being evaluated for inclusion in the
SIMD proposal.

Depends on D93771.

Differential Revision: https://reviews.llvm.org/D93775