[BranchRelaxation] Strengthen post condition assertions

[reames]

Summary

The whole point of this pass is to rewrite branches so that branches are in bounds. We should assert that we succeeded rather than just that we kept our internal data structure in sync.

[reames]

Add back faulting_op special case since my attempt at doing an NFC cleanup wasn't NFC on all targets.

[asb]

LGTM.

llvm/lib/CodeGen/BranchRelaxation.cpp

 #ifndef NDEBUG
   unsigned PrevNum = MF->begin()->getNumber();
   for (MachineBasicBlock &MBB : *MF) {
     const unsigned Num = MBB.getNumber();
     assert(!Num || BlockInfo[PrevNum].postOffset(MBB) <= BlockInfo[Num].Offset);
     assert(BlockInfo[Num].Size == computeBlockSize(MBB));
     PrevNum = Num;
   }
+
+  for (MachineBasicBlock &MBB : *MF) {
+    for (MachineBasicBlock::iterator J = MBB.getFirstTerminator();
+         J != MBB.end(); J = std::next(J)) {
+      MachineInstr &MI = *J;
+      if (!MI.isConditionalBranch() && !MI.isUnconditionalBranch())
+        continue;
+      if (MI.getOpcode() == TargetOpcode::FAULTING_OP)
+        continue;
+      MachineBasicBlock *DestBB = TII->getBranchDestBlock(MI);
+      assert(isBlockInRange(MI, *DestBB));
+    }
+  }
 #endif
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 /// print block size and offset information - debugging
 LLVM_DUMP_METHOD void BranchRelaxation::dumpBBs() {
   for (auto &MBB : *MF) {
     const BasicBlockInfo &BBI = BlockInfo[MBB.getNumber()];