flow: add a new expr->parent struct member

flow: add a new expr->parent struct member

I tried to use an expression_stack() for this, but it doesn't work.

The main place where this is used is to find for things like "&foo->bar".
The client sees "foo->bar" as a dereference, but if we look at the
->parent then we can see the '&' so we know it's not really a dereference.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>