Reactor backend: Subzero

Change-Id: I8e8af15a9b0406c9470a3e23d02345fe27740a51
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46448Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>