Ensure that basic blocks end with branch

[gavrilikhin-d]

Is your feature request related to a problem? Please describe.
Forgetting branch at the end of basic block is a common mistake, that leads to invalid IR.

Describe the solution you'd like
Add 2 builder classes for basic block:

1. Allows to insert any instruction until branch, then switches to second. If first builder is dropped produce a compile error, as IR is invalid.
2. Doesn't allow to insert any instruction, because we already have branch. No error on drop

Describe possible drawbacks to your solution
Must explicitly check for return or branch before adding an unknown instruction

Additional context
When I say "branch" I mean branch or return instruction.

[TheDan64]

Not sure that there's a good way to do this in inkwell. I think I always expected this sorta thing to be in a higher level abstraction ontop of inkwell. But it's worth exploring I suppose

[gavrilikhin-d]

Not sure that there's a good way to do this in inkwell. I think I always expected this sorta thing to be in a higher level abstraction ontop of inkwell. But it's worth exploring I suppose

I just thought that if inkwell adds type safety on top of llvm_sys, it may add "ir-safety" in some trivial cases as well

[TheDan64]

Indeed, we can if it makes sense to do so