Remove PointerIsValid()

I think there is agreement that the PointerIsValid() macro is pretty
useless. This patch proposes to remove it. There have been a few
recent mini-discussions in other threads that appear to support this. [0][1]

There were the usual concerns about code churn and backpatching and so
on, but I think in the end the change is not that big and it's in pretty
boring positions. Also, you can backpatch code without PointerIsValid()
just fine. You might run into trouble if you forward-patch. :-/

While converting the code, I tried to find a balance of style of

if (PointerIsValid(foo))

to

if (foo)

or

if (foo != NULL)

but there is no deterministic reason.

(Note that when you convert the first form to the third form, you have
to flip the overall sense of the logic, which might look confusing in
some places.)

[0]:
https://www.postgresql.org/message-id/CA+hUKG+NFKnr=K4oybwDvT35dW=VAjAAfiuLxp+5JeZSOV3nBg@mail.gmail.com
[1]:
https://www.postgresql.org/message-id/bccf2803-5252-47c2-9ff0-340502d5bd1c@iki.fi