Make Visitor more robust on instruction iteration

[xuechen417]

Use a fixed end guard for the instruction iteration is not robust for callbacks to add a basic block. Use Instruction::getNextNode() for the iteration to allow callbacks to directly edit the code.

[nhaehnle]

Thanks! That said, the previous code allowed the callback function to erase the given instruction (that's why it has the make_early_inc_range). So using getNextNode must happen before the call to visit.

Also, please fix the typo in the title: Visitor

[xuechen417]

Move getNextNode before calling visit

[xuechen417]

ping for review

[nhaehnle]

Thanks!