This change is mostly done to remove an unnecessary helper in
ReplaceVariable.cpp.

Tested locally, as the bots all support ES3.1+.  There are a few
failures, with or without this change.

Bug: angleproject:5556
Change-Id: I7bd2d4294c9dc164146d713db6b45cbc73e59c16
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2633437
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>