[WebAssembly] Prototype i8x16 to i32x4 widening instructions

As proposed in https://github.com/WebAssembly/simd/pull/395 and matching the
opcodes used in V8:
https://chromium-review.googlesource.com/c/v8/v8/+/2617385/4/src/wasm/wasm-opcodes.h

Differential Revision: https://reviews.llvm.org/D95557