[LCSSA] Add expensive verification of LCSSA form for sub-loops.

[fhahn]

Summary

This assertion makes sure all sub-loops are in LCSSA form before
bringing their parent in LCSSA form. This precondition was added to
formLCSSA in D56848.

[davide]

Yes, this was what I had in mind.

[davide]

Just please make sure to run stage3 build and verify this doesn't break.

[fhahn]

In D56921#1363424, @davide wrote:

Just please make sure to run stage3 build and verify this doesn't break.

Done, 3 stage bootstrap build of llvm/clang/libcxx and testing passed with the assertion.

[fhahn]

ping! Should we get this in?

[davide]

LGTM.

llvm/lib/Transforms/Utils/LCSSA.cpp

if (BlocksDominatingExits.insert(IDomBB))
       BBWorklist.push_back(IDomBB);
   }
 }
 
 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
                      ScalarEvolution *SE) {
   bool Changed = false;
 
+#ifdef EXPENSIVE_CHECKS
+  // Verify all sub-loops are in LCSSA form already.
+  for (Loop *SubLoop: L)
+    assert(SubLoop->isRecursivelyLCSSAForm(DT, *LI) && "Subloop not in LCSSA!");
+#endif
+
   SmallVector<BasicBlock *, 8> ExitBlocks;
   L.getExitBlocks(ExitBlocks);
   if (ExitBlocks.empty())
     return false;
 
   SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;
 
   // We want to avoid use-scanning leveraging dominance informations.