[LoopSimplifyCFG] Do not require MSSA. Continue to preserve if available.

[asbirlea]

Summary

LoopSimplifyCFG does not need MSSA, but should preserve it if it's available.

This is a legacy PM change, aimed to denoise the test changes in D109958.

[nikic]

LGTM

llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp

public:
 
   bool runOnLoop(Loop *L, LPPassManager &LPM) override {
     if (skipLoop(L))
       return false;
 
     DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
     ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
-    MemorySSA *MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
-    MemorySSAUpdater MSSAU(MSSA);
-    if (VerifyMemorySSA)
-      MSSA->verifyMemorySSA();
+    auto *MSSAA = getAnalysisIfAvailable<MemorySSAWrapperPass>();
+    Optional<MemorySSAUpdater> MSSAU;
+    if (MSSAA)
+      MSSAU = MemorySSAUpdater(&MSSAA->getMSSA());
+    if (MSSAA && VerifyMemorySSA)
+      MSSAU->getMemorySSA()->verifyMemorySSA();
     bool DeleteCurrentLoop = false;
-    bool Changed = simplifyLoopCFG(*L, DT, LI, SE, &MSSAU, DeleteCurrentLoop);
+    bool Changed = simplifyLoopCFG(
+        *L, DT, LI, SE, MSSAU.hasValue() ? MSSAU.getPointer() : nullptr,
+        DeleteCurrentLoop);
     if (DeleteCurrentLoop)
       LPM.markLoopAsDeleted(*L);
     return Changed;
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
-    AU.addRequired<MemorySSAWrapperPass>();
     AU.addPreserved<MemorySSAWrapperPass>();
     AU.addPreserved<DependenceAnalysisWrapperPass>();
     getLoopAnalysisUsage(AU);
   }
 };
 } // end namespace
 
 char LoopSimplifyCFGLegacyPass::ID = 0;