fixes #25208; generates a copy for opcLdConst in the assignments

[ringabout]

fixes #25208

type Conf = object
  val: int

const defaultConf = Conf(val: 123)
static:
  var conf: Conf
  conf = defaultConf

# opcLdConst is now always valid. We produce the necessary copy in the
# assignments now:

A opcLdConst is generated for defaultConf in conf = defaultConf. According to the comment above, we need to handle the copy for assignments of opcLdConst