fix Issue 7976 - ICE(backend/cg87.c)assignment btw two elements of dynamic array of complex number types

http://d.puremagic.com/issues/show_bug.cgi?id=7976

Will wait for https://github.com/D-Programming-Language/dmd/pull/2627 to be pulled before adding test case, so I don't have to spend a lot of time mucking about with merge conflicts.