You over-estimate how much LLVM might want Intel's code dumps.
Based on (minor) personal experience of gcc forks, it's not unusual to make some fairly significant change to some major data structure to make your CPU work better, but which would break several other backends.
There may not be any nice way of integrating these changes in a way which would make the acceptable upstream, without multi-months of work refactoring huge chunks of the compiler (which would still also need lots of work on all those other architectures, to make them compatible with your changes).
Based on (minor) personal experience of gcc forks, it's not unusual to make some fairly significant change to some major data structure to make your CPU work better, but which would break several other backends.
There may not be any nice way of integrating these changes in a way which would make the acceptable upstream, without multi-months of work refactoring huge chunks of the compiler (which would still also need lots of work on all those other architectures, to make them compatible with your changes).