The contract for TurboFan is that we use NumberConstants for any kind
of number value until the representation selection picks concrete
representations, i.e. Int32Constant or Float64Constant. We will soon
be able to also guard this contract with DCHECKs.

BUG=v8:5267
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2499573002
Cr-Commit-Position: refs/heads/master@{#40908}