[UnreachableMachineBlockElim] Enable machine verification after this pass

[foad]

Summary

Enabling this does not show any problems in check-llvm in an
LLVM_ENABLE_EXPENSIVE_CHECKS build.

[MatzeB]

I'm always happy when we can cover more parts of CodeGen with the machine verifier. Let's hope this sticks. LGTM

llvm/lib/CodeGen/TargetPassConfig.cpp

void TargetPassConfig::addOptimizedRegAlloc() {
   // FIXME: Once TwoAddressInstruction pass no longer uses kill flags,
   // LiveVariables can be removed completely, and LiveIntervals can be directly
   // computed. (We still either need to regenerate kill flags after regalloc, or
   // preferably fix the scavenger to not depend on them).
   // FIXME: UnreachableMachineBlockElim is a dependant pass of LiveVariables.
   // When LiveVariables is removed this has to be removed/moved either.
   // Explicit addition of UnreachableMachineBlockElim allows stopping before or
   // after it with -stop-before/-stop-after.
-  addPass(&UnreachableMachineBlockElimID, false);
+  addPass(&UnreachableMachineBlockElimID);
   addPass(&LiveVariablesID, false);
 
   // Edge splitting is smarter with machine loop info.
   addPass(&MachineLoopInfoID, false);
   addPass(&PHIEliminationID, false);
 
   // Eventually, we want to run LiveIntervals before PHI elimination.
   if (EarlyLiveIntervals)