Many transformations only look at the first variable in a
TIntermDeclaration.  This change validates that once the
SeparateDeclarations transformation has been run, multi-declarations are
not reintroduced.

Bug: angleproject:2733
Change-Id: If5f96689cbdd28817c479dd7c495b91535f87199
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2815565Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>