[CloneFunction] Don't crash if the value map doesn't hold something

It is possible for the value map to not have an entry for some value
that has already been removed.

I don't have a testcase, this is fall-out from a buildbot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277614 91177308-0d34-0410-b5e6-96231b3b80d8

Author: majnemer

lib/Transforms/Utils/CloneFunction.cpp

@@ -562,7 +562,7 @@ void llvm::CloneAndPruneIntoFromInst(Function *NewFunc, const Function *OldFunc,
   // Note that we must test the size on each iteration, the worklist can grow.
   for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
     const Value *OrigV = Worklist[Idx];
-    auto *I = cast<Instruction>(VMap.lookup(OrigV));
+    auto *I = cast_or_null<Instruction>(VMap.lookup(OrigV));
     if (!I)
       continue;