Temporary symbols will also be needed to store temporary arrays when complex
array expressions are unfolded.

Also clear tree update related structures at the end of updateTree(), so that
the traverser can be reused for several rounds of replacement more easily, and
remove unnecessary InVisit step from UnfoldShortCircuitToIf.

BUG=angleproject:971
TEST=angle_end2end_tests, WebGL conformance tests

Change-Id: Iecdd3008d43f01b02fe344ccde8614f70e6c0c65
Reviewed-on: https://chromium-review.googlesource.com/272121Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>