In the process, add a more general mechanism for passing around
and amending list of CodeStubAssembler Variables. That change
makes it possible to more easily add Variables to loops that are
generated by utility functions, e.g. BuildFastLoop.

LOG=N

Review-Url: https://codereview.chromium.org/2461363002
Cr-Commit-Position: refs/heads/master@{#40700}