- Add Simd128 type to Wasm AST types
 - Decode SIMD prefix, wasm opcodes correctly
 - Add a pass that converts SIMD machine ops to runtime calls
 - Sample opcodes Int32x4Splat, Int32x4ExtractLane and test

LOG=N
BUG=v8:4124

R=bradnelson@chromium.org, bbudge@chromium.org, titzer@chromium.org

Committed: https://crrev.com/73df92fc2fdbbfadc17e8ab4e58ec56ae2b3d91a
Review-Url: https://codereview.chromium.org/1991143002
Cr-Original-Commit-Position: refs/heads/master@{#37789}
Cr-Commit-Position: refs/heads/master@{#37807}