First step towards flow control.

Bug: b/128527271
Change-Id: I7e031ccc22148e37dc058150edc93d28de54f4c4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27096Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>