Explain why we still have kNumber in addition to kNumberOrOddball,
although the original motivation, which was Crankshaft, is gone now.

Bug: v8:7109
Change-Id: I33016fbfa96bb0db57473b6d0c720fa1389d11f1
Reviewed-on: https://chromium-review.googlesource.com/817439Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49991}