[WebAssembly] Narrowing and widening SIMD ops

Summary:
Implements target-specific LLVM intrinsics and clang builtins for
these new SIMD operations, as described at https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#integer-to-integer-narrowing.

Reviewers: aheejin

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D67425

llvm-svn: 371906