A while ago, CharacterRange::AddCaseEquivalents used to operate on a
single range (the `this` value) and add case equivalents for that to
`ranges`.

This was changed in a2baaaac to use `ranges` as a list of incoming
operands instead. When we now determine that the current range does not
have case equivalents, we need to `continue` instead of `return` to
avoid skipping the remaining ranges in the list.

Bug: v8:6940
Change-Id: I9face88a2ef8b9408f177e503f3399a25e688e06
Reviewed-on: https://chromium-review.googlesource.com/725430Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48682}