When the range of all non-bmp characters is passed to
AddUnicodeCaseEquivalents, icu::UnicodeSet::closeOver dutifully tries to
case-fold every single character in that range. Since we already know
this to be a nop, we can simply return instead.

This improves compilation time of /ui regexps by around 100x.

Bug: v8:6727
Change-Id: I79d73c77d6a54cbb5ad2cad0355214ed712b59b9
Reviewed-on: https://chromium-review.googlesource.com/635303
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47636}