[Constraint system] Move shrinking back into typeCheckExpression()

We only want this at the top level, not during the middle of constraint
generation.

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -3897,12 +3897,6 @@ bool ConstraintSystem::generateConstraints(
     SolutionApplicationTarget &target,
     FreeTypeVariableBinding allowFreeTypeVariables) {
   if (Expr *expr = target.getAsExpr()) {
-    // Try to shrink the system by reducing disjunction domains. This
-    // goes through every sub-expression and generate its own sub-system, to
-    // try to reduce the domains of those subexpressions.
-    shrink(expr);
-    target.setExpr(expr);
-
     // If the target requires an optional of some type, form a new appropriate
     // type variable and update the target's type with an optional of that
     // type variable.

lib/Sema/TypeCheckConstraints.cpp

@@ -2079,6 +2079,12 @@ TypeChecker::typeCheckExpression(
       target.getExprContextualTypeLoc(),
       target.getExprContextualTypePurpose());
 
+  // Try to shrink the system by reducing disjunction domains. This
+  // goes through every sub-expression and generate its own sub-system, to
+  // try to reduce the domains of those subexpressions.
+  cs.shrink(expr);
+  target.setExpr(expr);
+
   // If the client can handle unresolved type variables, leave them in the
   // system.
   auto allowFreeTypeVariables = FreeTypeVariableBinding::Disallow;