Fix #3812 and fix const of inplace assignments (#4065)

* Fix #3812 and fix const of inplace assignments

* Fix missing tests

* Revert operator overloading changes

* calculate answer first for tests

* Simplify tests

* Add more tests

* Add a couple more tests

* Add test_inplace_lshift, test_inplace_rshift for completeness.

* Update tests

* Shortcircuit on self assigment and address reviewer comment

* broaden skip for self assignment

* One more reviewer comment

* Document opt behavior and make consistent

* Revert unnecessary change

* Clarify comment

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>