Move AST transformations to compiler/translator/tree_ops.

BUG=angleproject:2409
TEST=angle_unittests

Change-Id: I9c620e98707d22d005da6192fe7d1b4e8030aadd
Reviewed-on: https://chromium-review.googlesource.com/975550Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>