RequirementMachine: Don't add unnecessary rule in merged associated types step

If we have a rewrite rule of the form

    X.[P:A] => X.[Q:A]

We introduce a pair of rules

    X.[P:A] => X.[P&Q:A]
    X.[Q:A] => X.[P&Q:A]

But in reality only the second one is necessary. The first one is redundant
because you obtain the same result by applying the original rule followed by
the second rule.

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -123,6 +123,10 @@ bool RewriteSystem::addRule(MutableTerm lhs, MutableTerm rhs) {
       lhs.back().getKind() == Symbol::Kind::AssociatedType &&
       rhs.back().getKind() == Symbol::Kind::AssociatedType &&
       lhs.back().getName() == rhs.back().getName()) {
+    if (Debug.contains(DebugFlags::Merge)) {
+      llvm::dbgs() << "## Associated type merge candidate ";
+      llvm::dbgs() << lhs << " => " << rhs << "\n\n";
+    }
     MergedAssociatedTypes.emplace_back(lhs, rhs);
   }