Teach llvm-flo how to handle two back-to-back JMPs

Summary:
If we have two consecutive JMP instructions and no branches to the
second one, the second one is dead code, but llvm-flo does not handle these
cases properly and put two JMPs in the same BB. This patch fixes this, putting
the extraneous JMP in a separate block, making it easy for us to detect it is
dead code and remove it later in a separate step.

(cherry picked from FBD2562465)