[WebAssembly] Implement i64x2.mul and remove i8x16.mul

Summary:
This reflects changes in the spec proposal made since basic arithmetic
was first implemented.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D80174