Pointer arithmetic compound assignments

[dgrunwald]

We don't support code like this:

someStruct.PointerField += i;
int* x = someStruct.PointerField++;

I'm not sure if it's worth adding support for these constructs -- these are two quite complex features interacting with each other; and such code is quite rare.

[dgrunwald]

Pointer compound assignments were implemented on #1021. However we still need more test cases (especially for pre/post-increment), so I'm keeping this issue open.

[dgrunwald]

Note: the "Help Wanted" here applies to the request for additional test cases.

The compound-assignment-transform is one of the more complicated parts of the decompiler and probably not a good starting point for new contributors.