[turbofan] Skip write barriers when storing smi.

[turbofan] Skip write barriers when storing smi.

On 64-bit targets, we can skip the write barrier for Store nodes if the
input is ChangeInt32ToTagged, because the value being stored is
definitely represented as a smi then.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/968113002

Cr-Commit-Position: refs/heads/master@{#26934}