add test for expr-level @:mergeBlock (closes #3921)

tests/unit/src/unit/issues/Issue3921.hx

@@ -0,0 +1,17 @@
+package unit.issues;
+
+class Issue3921 extends Test {
+    function test() {
+        eq(mb(), 3);
+    }
+
+    macro static function mb() {
+        return macro {
+            @:mergeBlock {
+                var v = 1;
+            }
+            v += 2;
+            v;
+        }
+    }
+}