[WebAssembly] Stop unrolling SIMD shifts since they are fixed in V8

Summary:
Fixes PR42973. Tests don't change because simd-arith.ll tests behavior
on unimplemented-simd128, which does not include any temporary
workarounds such as the one removed in this revision.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66166

llvm-svn: 368868