Non-feasible control-flow edges are currently removed by replacing the branch condition with a constant and then calling ConstantFoldTerminator. This happens in a rather roundabout manner, by inspecting the users (effectively: predecessors) of unreachable blocks, and further complicated by the need to explicitly materialize the condition for "forced" edges. I would like to extend SCCP to discard switch conditions that are non-feasible based on range information, but this is incompatible with the current approach (as there is no single constant we could use.)
Instead, this patch explicitly removes non-feasible edges. It currently only handles the case where there is a single feasible edge. The llvm_unreachable() branch will need to be implemented for the aforementioned switch improvement.