[REG2.064a] Issue 10726 - Bogus Circular Reference error if opEquals defined and has a loop

http://d.puremagic.com/issues/show_bug.cgi?id=10726

This is also the better fix for [issue 10567](http://d.puremagic.com/issues/show_bug.cgi?id=10567).

1. Move back `StructDeclaration::xeq` and `xcmp` generation after the semantic3 done
2. Stop duplicated TypeInfo generation for merely imported type declarations.