[analyzer] Allow TrackConstraintBRVisitor to work when the value it’s tracking is...

[analyzer] Allow TrackConstraintBRVisitor to work when the value it’s tracking is not live in the last node of the path

We always register the visitor on a node in which the value we are tracking is live and constrained. However,
the visitation can restart at a node, later on the path, in which the value is under constrained because
it is no longer live. Previously, we just silently stopped tracking in that case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179731 91177308-0d34-0410-b5e6-96231b3b80d8