Calls to `return()` in assembly are not interpreted as return nodes

For solc versions >= 0.6.0, assembly code is parsed and included in the CFG the same as typical Solidity code, except that explicit return statements are not parsed as `RETURN` type nodes. Given the following contract:
```solidity
contract Contract {
function foo(uint a) public returns (uint x) {
assembly {
switch a
case 0 { x := 0 }
case 1 { x := 1 }
default {
return(0x0, 32)
}
}
}
}
```
and using the implicit return handling from #1880, I get the following CFG, with the return statement highlighted in red:

Node 10 should really be a `NodeType.RETURN`, and probably should not have any sons. Node 11, at the bottom, is the implicit return node, which should not be reachable from the explicit return statement earlier inside the assembly block. There also probably shouldn't be a temporary variable for the value returned by `return()`, i.e., `TMP_2(None) = SOLIDITY_CALL return(uint256,uint256)(0,32)`. That is, unless my understanding of the return call in assembly is totally off.