[WebAssembly] SIMD bitmask intrinsics and builtin functions

Summary:
These experimental new instructions are proposed in
https://github.com/WebAssembly/simd/pull/201.

Reviewers: aheejin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76397