Partial fix for Issue 10567 - Typeinfo.compare has unreasonable signature requirements on opCmp

http://d.puremagic.com/issues/show_bug.cgi?id=10567

If needed, generate `__xopEquals` method in order to adapt `opCmp` member function (even if it's a template) to the function pointer `TypeInfo_Struct.xopCmp`.

BUG: Searching `opCmp` member doesn't work with alias this.