[ConstraintSolver] Fix `shrink` to use correct primary expression as a candidate
[ConstraintSolver] Fix `shrink` to use correct primary expression as a candidate

When trying to identify candidates for shrinking we are missing the case
when apply expression is a source of the assignment operator, which leads
to incorrect results in some situations, because shrink is going to miss
some required contextual information about assignment.

Resolves: rdar://problem/33190087

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