SILGen: Remove over-eager assertions in assignment lowering

We already assert that each element of the tuple is an lvalue;
asserting that the tuple itself has an lvalue or is Void is
incorrect because the tuple might have zero non-tuple elements
recursively but itself not be Void, eg ((), ()).

Fixes <https://bugs.swift.org/browse/SR-5919>.

Author: slavapestov

lib/SILGen/SILGenExpr.cpp

@@ -4283,9 +4283,6 @@ namespace {
 
     // If the destination is a tuple, recursively destructure.
     void visitTupleExpr(TupleExpr *E) {
-      auto *TTy = E->getType()->castTo<TupleType>();
-      assert(TTy->hasLValueType() || TTy->isVoid());
-      (void)TTy;
       for (auto &elt : E->getElements()) {
         visit(elt);
       }
@@ -4398,7 +4395,6 @@ static void emitSimpleAssignment(SILGenFunction &SGF, SILLocation loc,
 
   // Handle tuple destinations by destructuring them if present.
   CanType destType = dest->getType()->getCanonicalType();
-  assert(!destType->isMaterializable() || destType->isVoid());
 
   // But avoid this in the common case.
   if (!isa<TupleType>(destType)) {

test/SILGen/assignment.swift

@@ -49,3 +49,12 @@ func copyRightToLeft(p: inout P) {
   // CHECK:   end_access [[WRITE]] : $*P
   p.left = p.right
 }
+
+// SR-5919
+func stupidGames() -> ((), ()) {
+  return ((), ())
+}
+
+func assignToNestedVoid() {
+  let _: ((), ()) = stupidGames()
+}