[reg] fix Issue 13179 - AA key type TagIndex now requires equality rather than...

... comparison

This is a revert of https://github.com/D-Programming-Language/dmd/pull/3731

Considerable discussion revolved about this issue, unfortunately, no consensus was reached. See:

https://issues.dlang.org/show_bug.cgi?id=13074
https://issues.dlang.org/show_bug.cgi?id=13179
http://forum.dlang.org/post/mailman.212.1406134060.32463.digitalmars-d@puremagic.com