Depends on how well you keep your if's structured :).
The following expression does work with the CodeBlocks RegExp flavor and matches the presented format, it does allow extra whitespace where possible but it does not work with additional conditions nor if the if is spread across multiple lines nor if you use fancy unicode names for your identifiers (or anything else than 7-Bit ASCII).
Search: \([ \t]*if[ \t]*(\)[ \t]*![ \t]*\([a-zA-Z0-9_]+\.empty()\)[ \t]*\().*\)
Replace: \1\2 == false\3