Move ReplaceVariable to tree_util directory
This was overlooked earlier when most AST transform related utilities
were moved to tree_util.
BUG=angleproject:2402
TEST=angle_unittests
Change-Id: I17d3716a434f80a9316156ca019ce49aada5c249
Reviewed-on: https://chromium-review.googlesource.com/975881
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
