Multiple insertions to the same parent can be handled as long as the
insertions don't have the same position as well. They're sorted in
reverse order so that insertions to greater indices get processed
first.

This helps to make some AST transformations faster - they don't need
multiple tree traversals and updateTree() steps anymore. The
SimplifyLoopConditions AST transformation is changed to only use a
single traversal.

BUG=angleproject:1966
TEST=angle_unittests, angle_end2end_tests,
     WebGL conformance tests,
     dEQP-GLES2.functional.shaders.*select_iteration_count*

Change-Id: I3183f2644ad58b282926093c77b204fb7e4e9b71
Reviewed-on: https://chromium-review.googlesource.com/506202Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>