CodeGen: Avoid dereferencing end() in MachineScheduler

[dexonsmith]

Summary

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

It's possible that instead the loop here should check for I==RegionEnd
and continue to use I->isDebugValue() (instead of std::prev(I)), but I
doubt it.

This is trigged 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.

[MatzeB]

The change looks correct, as DebugValues should never be scheduling barriers I don't expect any differences in the compute NumRegionInstrs either.
LGTM.

[dexonsmith]

Thanks for the review! I committed this (but forgot to respond here until now) in r278394.

lib/CodeGen/MachineScheduler.cpp

for(MachineBasicBlock::iterator RegionEnd = MBB->end();
         --RegionEnd;
       }
 
       // The next region starts above the previous region. Look backward in the
       // instruction stream until we find the nearest boundary.
       unsigned NumRegionInstrs = 0;
       MachineBasicBlock::iterator I = RegionEnd;
       for (;I != MBB->begin(); --I) {
-        if (isSchedBoundary(&*std::prev(I), &*MBB, MF, TII))
+        MachineInstr &MI = *std::prev(I);
+        if (isSchedBoundary(&MI, &*MBB, MF, TII))
           break;
-        if (!I->isDebugValue())
+        if (!MI.isDebugValue())
           ++NumRegionInstrs;
       }
       // Notify the scheduler of the region, even if we may skip scheduling
       // it. Perhaps it still needs to be bundled.
       Scheduler.enterRegion(&*MBB, I, RegionEnd, NumRegionInstrs);
 
       // Skip empty scheduling regions (0 or 1 schedulable instructions).
       if (I == RegionEnd || I == std::prev(RegionEnd)) {