Re-apply the fix on DwarfEHPrepare and add a test (#2)

* Ad -run-twice to dwarf-eh-prepare.ll

* In DwarfEHPrepare, after all passes are run, RewindFunction may be a dangling
pointer to a dead function. To make sure it's valid, doFinalization nullptrs
RewindFunction just like the constructor and so it will be found on next run.

llvm-svn: 217737

Co-authored-by: Yaron Keren <yaron.keren@gmail.com>

https://reviews.llvm.org/D110979
(cherry picked from commit 8f6e49e8a3c0f31a55c342a2a41dda630508c5eb)

llvm/lib/CodeGen/DwarfEHPrepare.cpp

@@ -340,6 +340,11 @@ class DwarfEHPrepareLegacyPass : public FunctionPass {
     return prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM.getTargetTriple());
   }
 
+  bool doFinalization(Module &M) override {
+    RewindFunction = nullptr;
+    return false;
+  }
+
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<TargetPassConfig>();
     AU.addRequired<TargetTransformInfoWrapperPass>();