Remove duplication in c1 Block successor lists

The list of successors of a block in c1 seems to not have a single source of truth, but be duplicated in `BlockBegin::_successors` and `BlockEnd::_sux`. Removing this duplication would simplify a lot of verification code, and remove bug surface area.

Some hints that this is a true duplication:

c1_Instruction performs assertions on matching before element access:
```
inline int BlockBegin::number_of_sux() const { assert(_end == NULL || _end->number_of_sux() == _successors.length(), "mismatch"); return _successors.length(); }
inline BlockBegin* BlockBegin::sux_at(int i) const { assert(_end == NULL || _end->sux_at(i) == _successors.at(i), "mismatch"); return _successors.at(i); }
inline void BlockBegin::add_successor(BlockBegin* sux) { assert(_end == NULL, "Would create mismatch with successors of BlockEnd"); _successors.append(sux); }
```

And the _successor field is commented as "factor out later":
```
  // SSA specific fields: (factor out later)
  BlockList _successors; // the successors of this block
  BlockList _predecessors; // the predecessors of this block
  BlockList _dominates; // list of blocks that are dominated by this block
  BlockBegin* _dominator; // the dominator of this block
  // SSA specific ends
```

It's possible that all four of these fields should be factored out.