make distinct conversions addressable in VM

[metagn]

fixes #24097

For nkConv addresses where the conversion is between 2 types that are equal between backends, treat assignments the same as assignments to the argument of the conversion. In the VM this seems to be in genAsgn and genAsgnPatch, as evidenced by the special logic for nkDerefExpr etc.

This doesn't handle ranges after #24037 because sameBackendType is used and not sameBackendTypeIgnoreRange. This is so this is backportable without #24037 and another PR can be opened that implements it for ranges and adds tests as well. We can also merge sameBackendTypeIgnoreRange with sameBackendType since it doesn't seem like anything that uses it would be affected (only cycle checks and the VM), but then we still have to add tests.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 1fbb67f

Hint: mm: orc; opt: speed; options: -d:release
174432 lines; 8.159s; 654.336MiB peakmem