OpVariable handling needs to be split out; variables are clearly not
present in the type tree. Take care to apply the OpVariable's
decorations in the right place.

Bug: b/123779863

Change-Id: I2761a604683cf8b025f6b9ba1efa5383f53d7ed1
Reviewed-on: https://swiftshader-review.googlesource.com/c/24372Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>