Extend check_equality_for_exist2in() to cover inner table expressions

The function check_equality_for_exist2in() used for picking out equalities for the exists-to-in transformation only "Check if 'func' is an equality in form "inner_table.column = outer_expr"". Extend this so that it checks "inner_expr = outer_expr".

Note that there may be issues when inner_expr is a row, as for example

exists (select * from it where ((it.a, it.b), it.c) = ((ot.a, ot.b), ot.c))
=>
((ot.a, ot.b), ot.c) in (select ((it.a, it.b), it.c) from it)

is illegal because we can't select like this.

Thus for starters, perhaps limit it to scalar expressions only.