assigned_expr: handle fake parameter assignments

assigned_expr: handle fake parameter assignments

Say you have a function that does:

int frob(struct something *p)
{
	return p->x;
}

And the caller looks like:

	val = frob(p);

Then this gets split into val = p->x and val = frob().  In the original
code the val = frob(); assignment would overwrite the "val = p->x"
assignment.  The result ends up being &undefined, actually and not frob().
What a useless thing.

Save the "val = p->x" assignment.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>