[WebAssembly] Prototype prefetch instructions

As proposed in https://github.com/WebAssembly/simd/pull/352 and using the
opcodes used in the V8 prototype:
https://chromium-review.googlesource.com/c/v8/v8/+/2543167. These instructions
are only usable via intrinsics and clang builtins to make them opt-in while they
are being benchmarked.

Differential Revision: https://reviews.llvm.org/D93883