- Adds Int16x8, Int8x16 types.
- Adds neg, abs unary ops.
- Adds add, sub, mul, and signed / unsigned min / max, comparison ops.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/2638133002
Cr-Commit-Position: refs/heads/master@{#42674}