Reactor backend: Subzero

Change-Id: I713149fb04740c8addec729d4c41a9109a05dcca
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52008Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>