[mlir][Transforms] Fix CSE memEffectsCache handling for existing entries (NFC) (#192178)

The condition on detecting cache insertion was reversed. The consequence
was that we would always go through the path of "cache hit" first, but
find that the entry was the just-inserted one and then proceed with
updating it. Subsequent attempt could go through the "cache miss" part
and the cache would never be used.

Author: zackc6

mlir/lib/Transforms/CSE.cpp

@@ -196,7 +196,7 @@ bool CSEDriver::hasOtherSideEffectingOpInBetween(Operation *fromOp,
   Operation *nextOp = fromOp->getNextNode();
   auto result =
       memEffectsCache.try_emplace(fromOp, std::make_pair(fromOp, nullptr));
-  if (result.second) {
+  if (!result.second) {
     auto memEffectsCachePair = result.first->second;
     if (memEffectsCachePair.second == nullptr) {
       // No MemoryEffects::Write has been detected until the cached operation.