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.
