[LoopSimplify] Reduce amount of redundant SCEV invalidation (NFCI)

We are simplifying the loop and all its children. Each time, we
invalidate the top-most loop. The top-most loop is going to be
the same every time. The cost of SCEV invalidation is largely
independent from how data about the loop is actually cached, so
we should avoid redundant invalidations.

Author: nikic

llvm/lib/Transforms/Utils/LoopSimplify.cpp

@@ -693,12 +693,6 @@ static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
     }
   }
 
-  // Changing exit conditions for blocks may affect exit counts of this loop and
-  // any of its paretns, so we must invalidate the entire subtree if we've made
-  // any changes.
-  if (Changed && SE)
-    SE->forgetTopmostLoop(L);
-
   if (MSSAU && VerifyMemorySSA)
     MSSAU->getMemorySSA()->verifyMemorySSA();
 
@@ -737,6 +731,13 @@ bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
     Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
                                AC, MSSAU, PreserveLCSSA);
 
+  // Changing exit conditions for blocks may affect exit counts of this loop and
+  // any of its parents, so we must invalidate the entire subtree if we've made
+  // any changes. Do this here rather than in simplifyOneLoop() as the top-most
+  // loop is going to be the same for all child loops.
+  if (Changed && SE)
+    SE->forgetTopmostLoop(L);
+
   return Changed;
 }