Reactor backend: Subzero

Change-Id: I7da14621e8aaf5e6730e2a2b884ad09984310e25
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54828
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>