[WinEH] Moved funclet pads should be in relative order

We shifted the MachineBasicBlocks to the end of the MachineFunction in
DFS order.  This will not ensure that MachineBasicBlocks which fell
through to one another will remain contiguous.  Instead, implement
a stable sort algorithm for iplist.

This partially reverts commit r214150.

llvm-svn: 247978