[WebAssembly] SIMD shifts

Summary:
Implement shifts of vectors by i32. Since LLVM defines shifts as
binary operations between two vectors, this involves pattern matching
on splatted shift operands. For v2i64 shifts any i32 shift operands
have to be zero extended in the input and any i64 shift operands have
to be wrapped in the output. Depends on D52007.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D51906

llvm-svn: 342302