[unroll] Strengthen verification of analysis updates under expensive asserts

I am suspecting a bug around updates of loop info for unreachable exits, but don't have a test case.  Running this locally on make check didn't reveal anything, we'll see if the expensive checks bots find it.

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -99,6 +99,17 @@ UnrollVerifyDomtree("unroll-verify-domtree", cl::Hidden,
 #endif
                     );
 
+static cl::opt<bool>
+UnrollVerifyLoopInfo("unroll-verify-loopinfo", cl::Hidden,
+                    cl::desc("Verify loopinfo after unrolling"),
+#ifdef EXPENSIVE_CHECKS
+    cl::init(true)
+#else
+    cl::init(false)
+#endif
+                    );
+
+
 /// Check if unrolling created a situation where we need to insert phi nodes to
 /// preserve LCSSA form.
 /// \param Blocks is a vector of basic blocks representing unrolled loop.
@@ -764,6 +775,9 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
   // Apply updates to the DomTree.
   DT = &DTU.getDomTree();
 
+  assert(!UnrollVerifyDomtree ||
+         DT->verify(DominatorTree::VerificationLevel::Fast));
+
   // At this point, the code is well formed.  We now simplify the unrolled loop,
   // doing constant propagation and dead code elimination as we go.
   simplifyLoopAfterUnroll(L, !CompletelyUnroll && ULO.Count > 1, LI, SE, DT, AC,
@@ -777,6 +791,10 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
   if (CompletelyUnroll)
     LI->erase(L);
 
+  // LoopInfo should not be valid, confirm that.
+  if (UnrollVerifyLoopInfo)
+    LI->verify(*DT);
+
   // After complete unrolling most of the blocks should be contained in OuterL.
   // However, some of them might happen to be out of OuterL (e.g. if they
   // precede a loop exit). In this case we might need to insert PHI nodes in