This is to be able to catch issues introduced through AST manipulation
earlier than shader compilation time, improving debuggability.

Bug: angleproject:2733
Change-Id: Ic57bc72f2ab438e60f32553d602074f3d72cc4f5
Reviewed-on: https://chromium-review.googlesource.com/c/1199922Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>