Assert on SSA errors in LiveVariables.

All uses of a virtual register must be dominated by its def.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152449 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveVariables.cpp

@@ -109,6 +109,7 @@ void LiveVariables::MarkVirtRegAliveInBlock(VarInfo& VRInfo,
   // Mark the variable known alive in this bb
   VRInfo.AliveBlocks.set(BBNum);
 
+  assert(MBB != &MF->front() && "Can't find reaching def for virtreg");
   WorkList.insert(WorkList.end(), MBB->pred_rbegin(), MBB->pred_rend());
 }