[dmd-internals] Critical code gen fixes for dmd

[Walter Bright via dmd-internals]

https://github.com/D-Programming-Language/dmd/pull/3624
https://github.com/D-Programming-Language/dmd/pull/3622

Code gen bugs are some of the worst bugs in a compiler in their adverse affect 
on users. These need to get pulled. I understand that reviewing the intricacies 
of register allocation, etc., in a compiler is not at all easy. But there really 
isn't anyone other than me who knows how it works in dmd.

The sooner it gets pulled, the less likely it is to break code in the next release.