- Adds some unit tests for InstructionSelector::TryMatch* methods.
- Adds a TryMatchIdentity method. We should detect identity shuffles
  and emit no code in that case.

Bug: v8:6020
Change-Id: I5dea84738bf87db7112eb7d19f91b1e6b20811c7
Reviewed-on: https://chromium-review.googlesource.com/1116058
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54097}