Instead of having the compiler do any validation on the
graph it has created instead have the compiler just compile
and have the engine that uses that compiled result do any
post compilation validation instead.
This makes it more clear that the compiler just compiles a
flow (and tasks and nested flows) into a graph, and that is
all that it does.
Change-Id: I96a35d732dc2be9fc8bc8dc6466256a19ac2df6d