The basic idea is to replace the current SSBO output traversal by build
a new AST for the Load and Store subtrees, then traverse that AST with
the regular OutputHLSL traversal.

This way, all the paren balancing and state management can go away.

BUG: angleproject:5734
Change-Id: I14d2fecd724d7419fbc9315fad4e9202a741b03f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2773401
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>