[WebAssembly] Initial SIMD128 support.

Kicks off the implementation of wasm SIMD128 support (spec:
https://github.com/stoklund/portable-simd/blob/master/portable-simd.md),
adding support for add, sub, mul for i8x16, i16x8, i32x4, and f32x4.

The spec is WIP, and might change in the near future.

Patch by João Porto

Differential Revision: https://reviews.llvm.org/D22686

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277543 91177308-0d34-0410-b5e6-96231b3b80d8
17 files changed