`compare_key`, introduced in #42409, previously supported only `=` and `LIKE`
operators. This changeset introduces a number of other operators: `!=`, `IN`,
`NOT IN`, `NOT LIKE`, `RLIKE`, `REGEXP`, `NOT REGEXP`, `EXISTS`, and `NOT EXISTS`
(the latter two aliased to `=` and `!=`, respectively). To support case-sensitive
regular expression key comparisons, the new `type_key` parameter will force
a MySQL `CAST` when 'BINARY' is passed.
Props soulseekah.
Fixes#43346.
git-svn-id: https://develop.svn.wordpress.org/trunk@46188 602fd350-edb4-49c9-b593-d223f7449a82
6eabc83b81