RequirementMachine: Fix assertion in mergeAssociatedTypes()

We used to assert that the merged protocol set was larger than or
equal to in size to both the left hand side and the right hand
side.

However, there is a counter-example:

    protocol P {}
    protocol Q {}
    protocol R : P, Q {}

    LHS = [P&Q:T], RHS = [R:T]

The merged atom is `[R:T]` which has fewer protocols than the
left hand side.

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystemCompletion.cpp

@@ -202,9 +202,9 @@ Atom RewriteSystem::mergeAssociatedTypes(Atom lhs, Atom rhs) const {
   }
 
   // The two input sets are minimal already, so the merged set
-  // should have at least as many elements as each input set.
-  assert(minimalProtos.size() >= protos.size());
-  assert(minimalProtos.size() >= otherProtos.size());
+  // should have at least as many elements as the smallest
+  // input set.
+  assert(minimalProtos.size() >= std::min(protos.size(), otherProtos.size()));
 
   // The merged set cannot contain more elements than the union
   // of the two sets.