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>