[x86] Support immediate indices for StoreWriteBarrier.

[x86] Support immediate indices for StoreWriteBarrier.

Ideally we would not need the StoreWriteBarrier instructions at all,
but represent the RecordWrite functionality as machine subgraph, but
that'll take some time to get there. In the mean time we can have a
shorter instruction sequence on Intel platforms by recognizing immediate
indices here.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1075103002

Cr-Commit-Position: refs/heads/master@{#27731}