Reactor backend: Subzero

Change-Id: I0e14ea808524eea31d4c12aa501cd2e98e445521
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51768Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>