In AccessEnforcementOpts, skip SCC analysis.

When there is nothing that can be merged, there's no point doing an
extra CFG analysis.

Author: atrick

lib/SILOptimizer/Transforms/AccessEnforcementOpts.cpp

@@ -949,6 +949,10 @@ canMerge(PostDominanceInfo *postDomTree,
 static bool mergeAccesses(
     SILFunction *F, PostDominanceInfo *postDomTree,
     const AccessConflictAndMergeAnalysis::MergeablePairs &mergePairs) {
+
+  if (mergePairs.empty())
+    return false;
+
   bool changed = false;
 
   // Compute a map from each block to its SCC -