AD-curve, just like before, displays combinations of Y and P where both goods market and money market are in equilibrium. At any given instance, even when we have inflation, aggregate demand would as before depend negatively on P. Description, as follows is little more involved. Let's say that price level one year ago was 100 and that P is price level today. Then p = (P - 100)/100 is rate of inflation during the previous year and P = (1 + p).100 today. For illustration if p is 10%, we have P = (1 + 0.1) .100 = 110 today. Given that price level in the previous year, we have a positive relationship between P and p.
Given price level last year, there is a price level today that would make inflation exactly same as the growth rate in money supply over the last year. For instance, say that pM was 4% in previous year and P was 100 a year ago then if P = 104 today we have p = pM, IS- and LM-curves are stable and we can find level of GDP that gives the equilibrium in both markets by finding the point where they intersect.
Now, to display that AD curve slopes downwards, we should demonstrate that if P > 104, a lower level of GDP will lead to simultaneous equilibrium. To see this, just note that for P > 104, inflation has been a little higher and LM curve will be a little higher up resulting in a lower level of GDP. A similar argument demonstrates that GDP should be higher if P < 104 for both markets to remain in equilibrium.
So at a given point in time, given the price level last year, aggregate demand would still rely negatively on P and the AD curve will slope downwards.