Reactor backend: Subzero

Change-Id: I5d78eb4b823a056a07be57932c51391661d4f845
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47268Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>