fix Issue 10040 - struct-related ICE

The second instantiation of `mixinMembers!(MsgHeader.init)` is incorrectly
judged as not identical with the first one, by the lack of `StructLiteralExp::equals`.
Merge pull request #2000 from 9rnsr/fix10040

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