Rework constructor handling (#446)

tests/compiler/std/new.untouched.wat

@@ -101,27 +101,23 @@
   return
  )
  (func $std/new/AClass#constructor (; 2 ;) (type $ifi) (param $0 i32) (param $1 f32) (result i32)
-  (local $2 i32)
   get_local $0
-  get_local $0
-  if (result i32)
+  block (result i32)
    get_local $0
-  else   
-   block (result i32)
+   i32.eqz
+   if
     i32.const 8
     call $~lib/memory/memory.allocate
-    set_local $2
-    get_local $2
-    i32.const 1
-    i32.store
-    get_local $2
-    f32.const 2
-    f32.store offset=4
-    get_local $2
+    set_local $0
    end
-   tee_local $0
+   get_local $0
+   i32.const 1
+   i32.store
+   get_local $0
+   f32.const 2
+   f32.store offset=4
+   get_local $0
   end
-  tee_local $0
   i32.load
   i32.const 1
   i32.add