NewTransition returns a Transition. context is the valuecontext of the "start" node. callSite is the ssa.Function which "goes" to the other context. targetContext is the valuecontext which is the "goal" of the flow. NewTransition appends the new transition to the transitions slice.
(context *ValueContext, targetContext *ValueContext, node ssa.Instruction)
source not stored for this graph (policy: none)