Adapt to new special-ref result variables in out contracts

The AST now features special-ref result variables (storage classes:
ref, temp, result) after rewriting out contracts; from dmd/func.d:

/*   out(id1) { statements1... }
 *   out(id2) { statements2... }
 *   ...
 * becomes:
 *   out(__result) { { ref id1 = __result; { statements1... } }
 *                   { ref id2 = __result; { statements2... } } ... }
 */

We are talking about the `id1` and `id2` variables here.
There's an existing assertion that we don't set a special-ref variable's
lvalue (T**) to the sret pointer (T*) which was already triggered when
compiling Phobos without unittests.

gen/llvmhelpers.cpp

@@ -966,9 +966,10 @@ void DtoVarDeclaration(VarDeclaration *vd) {
 
   if (isIrLocalCreated(vd)) {
     // Nothing to do if it has already been allocated.
-  } else if (gIR->func()->sretArg && ((gIR->func()->decl->nrvo_can &&
-                                       gIR->func()->decl->nrvo_var == vd) ||
-                                      vd->isResult())) {
+  } else if (gIR->func()->sretArg &&
+             ((gIR->func()->decl->nrvo_can &&
+               gIR->func()->decl->nrvo_var == vd) ||
+              (vd->isResult() && !isSpecialRefVar(vd)))) {
     // Named Return Value Optimization (NRVO):
     // T f() {
     //   T ret;        // &ret == hidden pointer