Fix assignment of return variables from modifiers.

test/libsolidity/semanticTests/modifiers/function_modifier_return_reference.sol

@@ -0,0 +1,15 @@
+contract C {
+    modifier m1(uint value) {
+        _;
+    }
+    modifier m2(uint value) {
+        _;
+    }
+
+    function f() public m1(x = 2) m2(y = 3) returns (uint x, uint y) {
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 2, 3