[WebAssembly] Expand SIMD shifts while V8's implementation disagrees

Summary:
V8 currently implements SIMD shifts as taking an immediate operation,
which disagrees with the spec proposal and the toolchain
implementation. As a stopgap measure to get things working, unroll all
vector shifts. Since this is a temporary measure, there are no tests.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D56520

llvm-svn: 351151