how can I partition a set of instructions in IR to a new file?

Question

I want to partition a set of instructions in IR to a new file (like .bc). The process would be:

1) copy the instructions

2) remove the instructions from the original file

3) dump it to a new file like (.bc)

it seems like 1) and 2) can be solved by " In LLVM IR, I want to copy a set of Instructions and paste those instructions to another place in IR through LLVM pass. How to do this? "

but, how can I deal with the 3) ?

Thanks in advance!