[WebAssembly] Prototype extending multiplication SIMD instructions

As proposed in https://github.com/WebAssembly/simd/pull/376. This commit
implements new builtin functions and intrinsics for these instructions, but does
not yet add them to wasm_simd128.h because they have not yet been merged to the
proposal. These are the first instructions with opcodes greater than 0xff, so
this commit updates the MC layer and disassembler to handle that correctly.

Differential Revision: https://reviews.llvm.org/D90253