[REG2.063a] Issue 10040 - struct-related ICE

http://d.puremagic.com/issues/show_bug.cgi?id=10040

This is one of the AST-identity issue. Maybe GDC/LDC requires this fix for the particular code compilation.

The second instantiation of `mixinMembers!(MsgHeader.init)` is incorrectly
judged as not identical with the first one, by the lack of `StructLiteralExp::equals`.