Assign consistent symbol ids to out base, index and value nodes that
get created in RemoveDynamicIndexing. Some utility functions in
RemoveDynamicIndexing have also now become unnecessary now that there
are better generic helpers and node constructors.

BUG=angleproject:1490
TEST=angle_unittests, angle_end2end_tests

Change-Id: I98891e448b85d51518dbf3156ec93f9661def57e
Reviewed-on: https://chromium-review.googlesource.com/580954Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>