I need to also walk interface objects in shader prolog & epilog; will
reuse this logic.

Bug: b/124388146
Change-Id: Ida735f58aa1bc36d83e4a6ea3c16925a9c5656b0
Reviewed-on: https://swiftshader-review.googlesource.com/c/24808Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>