Fix unsound transform pass (#8633)

When a `var openArray[T]` function parameter goes trough the
`transformAddrDeref` pass we may lose the `var` specifier, leading to
nasty crashes at runtime.

tests/ccgbugs/topenarraycast.nim

@@ -0,0 +1,8 @@
+proc foo[T](s: var openArray[T]): T =
+  for x in s: result += x
+
+proc bar(xyz: var seq[int]) =
+  doAssert 6 == (seq[int](xyz)).foo()
+
+var t = @[1,2,3]
+bar(t)