- Add Simd128 type to Wasm AST types
 - Add a pass that converts SIMD machine ops to runtime calls
 - Sample opcodes Int32x4Splat, Int32x4ExtractLane and test
 - Separate out generic SIMD Machine ops as these cannot be
 handled by runtime functions just yet.

LOG=N
BUG=v8:4124

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

Review-Url: https://codereview.chromium.org/1991143002
Cr-Commit-Position: refs/heads/master@{#37789}