[WebAssembly] SIMD integer abs instructions

Summary:
These were merged to the SIMD proposal in
https://github.com/WebAssembly/simd/pull/128.

Depends on D76397 to avoid merge conflicts.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76399