The FeedbackNexus classes initially were one-to-one with IC classes,
but over time this got out of date. We also found Nexus' useful, so
we made more classes even for cases that weren't ICs.

The inheritence and polymorphism became confusing and led to
duplication. Better, to just talk about a (single) FeedbackNexus.

Bug: v8:7344
Change-Id: I509dc9657895d56c3859de6e6589695cdff9e73e
Reviewed-on: https://chromium-review.googlesource.com/890452
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50997}