This patch uses deepCopy for nodes which are used more than once in the SSBO
traverser.

Bug: angleproject:1951
Change-Id: Ie8e03d4f595484a1e2ca0397ed30a7f484add607
Reviewed-on: https://chromium-review.googlesource.com/c/1370678Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>