ES31: Use deepCopy to make sure that every node being used only once 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/1370678 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>