Remove meaningless assert on debug build

Additional fix for issue 11233
fix Issue 11257 - Allow whole implicit conversion if one or more overlapped field could.
fix Issue 9665 - Structure constant members can not be initialized if have opAssign

- Change the first field assignment inside constructor to true initialization.
- Disable multiple initialization of non-mutable field, if it is once initialized.
fix Issue 11246 - [REG 2.063] Struct initialized in constructor is destroyed first
Merge pull request #2665 from 9rnsr/fix9665

Issue 9665 - Structure constant members can not be initialized if have opAssign