Create temp var in deepcopy if needed (#5205)

tests/ccgbugs/tdeepcopy_addr_rval.nim

@@ -0,0 +1,16 @@
+discard """
+  output: "3"
+"""
+
+# issue 5166
+
+type
+  Test = ref object
+    x: int
+
+let x = Test(x: 3)
+let p = cast[pointer](x)
+
+var v: Test
+deepCopy(v, cast[Test](p))
+echo v.x