PR9420; an instruction before an unreachable is guaranteed not to have any

reachable uses, but there still might be uses in dead blocks.  Use the
standard solution of replacing all the uses with undef.  This is
a rare case because it's very sensitive to phase ordering in SimplifyCFG.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127299 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2168,7 +2168,9 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
       if (LI->isVolatile())
         break;
     
-    // Delete this instruction
+    // Delete this instruction (any uses are guaranteed to be dead)
+    if (!BBI->use_empty())
+      BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
     BBI->eraseFromParent();
     Changed = true;
   }