Identify simple column references in extended statistics

Until now, when defining extended statistics, everything except a plain
column reference was treated as complex expression. So for example "a"
was a column reference, but "(a)" would be an expression. In most cases
this does not matter much, but there were a couple strange consequences.
For example

    CREATE STATISTICS s ON a FROM t;

would fail, because extended stats require at least two columns. But

    CREATE STATISTICS s ON (a) FROM t;

would succeed, because that requirement does not apply to expressions.
Moreover, that statistics object is useless - the optimizer will always
use the regular statistics collected for attribute "a".

So do a bit more work to identify those expressions referencing a single
column, and translate them to a simple column reference. Backpatch to
14, where support for extended statistics on expressions was introduced.

Reported-by: Justin Pryzby
Backpatch-through: 14
Discussion: https://postgr.es/m/20210816013255.GS10479%40telsasoft.com

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