This adds support for lowering keyed access to JSTypedArray objects to
element loads and stores instead of IC calls. There's still a lot of
room for improvement, but the improvements can be done incrementally
later.

We add a dedicated UnsafePointerAdd operator, which sits in the effect
chain, and does the (GC invisible) computation of addresses that are
potentially inside HeapObjects. Also there's now a dedicated Retain
operator, which ensures that we retain a certain tagged value, which is
necessary to ensure that we keep a JSArrayBuffer alive as long as we
might still potentially access elements in its backing store.

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2203693002
Cr-Commit-Position: refs/heads/master@{#38235}