- Adds vqadd.s/u, vqsub.s/u for all integer lane sizes.
- Refactors disassembler and simulator, using switches instead
of long if-else chains.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/2649323012
Cr-Commit-Position: refs/heads/master@{#42865}