Refactor more of the migration tool to use NullabilityNode rather than ConstraintVariable.

With this CL, the NullabilityNode class is now used for the following
purposes:

- In ConditionalDiscard, to keep track of when branches of "if"
  statements should be discarded.

- To track the nullability relationships implied by an assignment.

- To track the nullability implied by an optional parameter that lacks
  a default value.

- In _ConditionInfo, to keep track of how assert statements imply
  non-null intent.

Change-Id: Ieb05bb569258a3d16720c127a3f6038f805304c3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/98960
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>