rewrite `nvl/ifnull` to `nvl2`

[guojidan]

          if this pr is merged, we can rewrite `nvl/ifnull` function to `nvl2`, like:

nvl(expr1, expr2) -> nvl2(expr1, expr1, expr2)

🤔

Originally posted by @guojidan in #9364 (comment)

[guojidan]

take

[Jefffrey]

nvl has been rewritten to be a thin wrapper over coalesce now: #17991

So I don't think this is needed anymore