Relax the linear flow symbol constraints

In order to make it possible to have a symbol
tree we need to relax and remove the constraints
that are being imposed by the linear constraints
and later move those constraint checks and validations
into the engines compilation stage.

Part of blueprint taskflow-improved-scoping

Change-Id: I6efdc821ff991e83572d89f56be5c678d007f9f8
Merge "Relax the linear flow symbol constraints"