_aaEq was added to runtime/internal/aaA.d which forwards to
TypeInfo_AssociativeArray.equals in genobj.d. On the codegen side, DtoAAEquals
was added to gen/aa.cpp and is called from EqualExp::toElem in gen/toir.cpp.
I assume that the frontend will produce an error if == is used on associative
arrays of different type.
This fixes DMD bug 1429.