SILGen: Remove code that became dead from Swift 3 removal

Swift 3 mode has not been fully removed, but the constraint solver
support for the old function argument tuple behavior is gone, so
rip out some SILGen hacks for it too.

Author: slavapestov

lib/SILGen/SILGenExpr.cpp

@@ -2566,14 +2566,7 @@ RValue RValueEmitter::visitTupleShuffleExpr(TupleShuffleExpr *E,
   // If we're emitting into an initialization, we can try shuffling the
   // elements of the initialization.
   if (Initialization *I = C.getEmitInto()) {
-    // In Swift 3 mode, we might be stripping off labels from a
-    // one-element tuple; the destination type is a ParenType in
-    // that case.
-    //
-    // FIXME: Remove this eventually.
-    if (I->canSplitIntoTupleElements() &&
-        !(E->getType()->hasParenSugar() &&
-          SGF.getASTContext().isSwiftVersion3())) {
+    if (I->canSplitIntoTupleElements()) {
       emitTupleShuffleExprInto(*this, E, I);
       return RValue::forInContext();
     }
@@ -2590,24 +2583,6 @@ RValue RValueEmitter::visitTupleShuffleExpr(TupleShuffleExpr *E,
   // Prepare a new tuple to hold the shuffled result.
   RValue result(E->getType()->getCanonicalType());
 
-  // In Swift 3 mode, we might be stripping off labels from a
-  // one-element tuple; the destination type is a ParenType in
-  // that case.
-  //
-  // FIXME: Remove this eventually.
-  if (E->getType()->hasParenSugar() &&
-      SGF.getASTContext().isSwiftVersion3()) {
-    assert(E->getElementMapping().size() == 1);
-    auto shuffleIndex = E->getElementMapping()[0];
-    assert(shuffleIndex != TupleShuffleExpr::DefaultInitialize &&
-           shuffleIndex != TupleShuffleExpr::CallerDefaultInitialize &&
-           shuffleIndex != TupleShuffleExpr::Variadic &&
-           "Only argument tuples can have default initializers & varargs");
-
-    result.addElement(std::move(elements[shuffleIndex]).ensurePlusOne(SGF, E));
-    return result;
-  }
-
   auto outerFields = E->getType()->castTo<TupleType>()->getElements();
   auto shuffleIndexIterator = E->getElementMapping().begin();
   auto shuffleIndexEnd = E->getElementMapping().end();