Replace `getNext` with `nextNonMarker`

Most of the times, when we need to get the next instruction, we actually
want to skip over "marker" function calls (e.g., calls to `newpc` and
`function_call`). `nextNonMarker` does exactly this.

`FunctionCallIdentification::isCall` and `JumpTargetManager::setCFGForm`
have also been extended to correctly handle such situations.