[MachineFunction] Introduce a reset method.

[MachineFunction] Introduce a reset method.

This method allows to reset the state of a MachineFunction as if it was
just created. This will be used during the bring-up of GlobalISel to
provide a way to fallback on SelectionDAG. That way, we can start doing
correctness testing even if we are not able to select all functions via
the global instruction selector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279876 91177308-0d34-0410-b5e6-96231b3b80d8