Some transformations change variable declarations.  This validation
ensures that all references to said variables are appropriately replaced
as well.

Bug: angleproject:2733
Change-Id: I6c2873968eeed4cba66e70069f84eb69a1f77074
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2818140
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>