[WebAssembly] Add experimental SIMD dot product instruction
Summary:
This instruction is not merged to the spec proposal, but we need it to
be implemented in the toolchain to experiment with it. It is available
only on an opt-in basis through a clang builtin.
Defined in https://github.com/WebAssembly/simd/pull/127.
Depends on D69696.
Reviewers: aheejin
Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, cfe-commits, llvm-commits
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D69697