[WebAssembly] Implement prototype SIMD rounding instructions

Summary:
As specified in https://github.com/WebAssembly/simd/pull/232. These
instructions are implemented as LLVM intrinsics for now rather than
normal ISel patterns to make these instructions opt-in. Once the
instructions are merged to the spec proposal, the intrinsics will be
replaced with proper ISel patterns.

Reviewers: aheejin

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D81222