plpgsql unreachable code (was BUG #1329: Bug in IF-ELSEIF-ELSE construct)

Neil Conway wrote:
> (BTW, another thing this example exposes is that we don't issue warnings
> about trivially-dead-code, such as statements in a basic block that
> follow a RETURN. This would probably be also worth doing.)

Attached is a patch that implements this. Specifically, if there are any
statements in the same block that follow a RETURN, EXIT (without
condition) or RAISE EXCEPTION statement, we issue a warning at CREATE
FUNCTION time:

create function exit_warn() returns int as $$
declare x int;
begin
x := 5;
loop
x := x + 1;
exit;
x := x + 2;
end loop;
x := x + 3;
return x;
end;$$ language 'plpgsql';
WARNING: assignment is unreachable, due to exit near line 6
CONTEXT: compile of PL/pgSQL function "exit_warn" near line 7

No warning is issued if check_function_bodies is false.

AFAICS there is no current infrastructure for walking a PL/PgSQL
function's parse tree, so I did this manually (which is easy enough, of
course). In the future it might be a good idea to refactor this to use
something akin to the "walker" infrastructure in the backend (for one
thing the PL/PgSQL function dumping code could use this as well).

(BTW this patch is intended for 8.1, of course.)

-Neil