Remove unreferenced variables from the AST Unreferenced local and global variables are now pruned from the AST. They will be removed unless their initializer has side effects. The CollectVariables step needs to be run after the pruning, as the pruning may affect which interface variables are statically used. It's also good to gather built-ins that need to be emulated after the pruning, so unnecessary built-in emulation functions are not added to the translator output. This will help handle some dEQP tests for arrays of arrays that have extremely large local arrays that are only used in an array length query. By constant folding the length and pruning unused variables we will avoid adding a large amount of array initialization code to the generated shaders. BUG=angleproject:2166 TEST=angle_unittests, angle_end2end_tests Change-Id: Ic918bfe8f16460bcd6101d73a7a674145f5aeecd Reviewed-on: https://chromium-review.googlesource.com/766434 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>