Reactor backend: LLVM

Change-Id: Id3e18f2779facfe3f441f684a40540a86d904643
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54489Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>