[ARM] Use a Changed flag to avoid making a pass's return value dependent on a compare with a Statistic object.

Statistic compile to always be 0 in release build so this compare would always return false. And in the debug builds Statistic are global variables and remember their values across pass runs. So this compare returns true anytime the pass runs after the first time it modifies something.

This was found after reviewing all usages of comparison operators on a Statistic object. We had some internal code that did a compare with a statistic that caused a mismatch in output between debug and release builds. So we did an audit out of paranoia.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302450 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/ARMOptimizeBarriersPass.cpp

@@ -88,13 +88,15 @@ bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
       }
     }
   }
+  bool Changed = false;
   // Remove the tagged DMB
   for (auto MI : ToRemove) {
     MI->eraseFromParent();
     ++NumDMBsRemoved;
+    Changed = true;
   }
 
-  return NumDMBsRemoved > 0;
+  return Changed;
 }
 
 /// createARMOptimizeBarriersPass - Returns an instance of the remove double