CodeGen: Avoid dereferencing end() in MachineScheduler

Check MachineInstr::isDebugValue for the same instruction as we're
calling isSchedBoundary, avoiding the possibility of dereferencing
end().

This is a functionality change even when I!=end().  Matthias had a look
and agrees this is the right resolution (as opposed to checking for
end()).

This is triggered by a huge number of tests, but they happen to
magically pass right now.  I found this because WIP patches for PR26753
convert them into crashes.

llvm-svn: 278394