[Relax][BugFix] Fix a bug about the IR construction in test file (#17121)

Update test_transform_dead_code_elimination.py

Fix the wrong Relax IR construction

Author: Cookiee235

tests/python/relax/test_transform_dead_code_elimination.py

@@ -454,7 +454,7 @@ def main(
                 R.output(lv0)
 
             gv_x = R.astype(x, dtype="float16")
-            gv_w = R.astype(x, dtype="float16")
+            gv_w = R.astype(w, dtype="float16")
 
             with R.dataflow():
                 lv1: R.Tensor((2, 28, 28, 3), dtype="float16") = R.permute_dims(
@@ -481,7 +481,7 @@ def main(
             w: R.Tensor((4, 3, 3, 3), dtype="float32"),
         ):
             gv_x = R.astype(x, dtype="float16")
-            gv_w = R.astype(x, dtype="float16")
+            gv_w = R.astype(w, dtype="float16")
 
             with R.dataflow():
                 lv1: R.Tensor((2, 28, 28, 3), dtype="float16") = R.permute_dims(