previously declared variable fails on DirectX.

Combined user defined variables with their unique ids to avoid
overwriting same name variables of different scope.

Bug: angleproject:2126
Change-Id: If9ad9e48f629d83b105d43ee28a50b8176d0e0a1
Reviewed-on: https://chromium-review.googlesource.com/c/1456484Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>