AliasAnalysis: speed up canApplyDecrementRefCount() for large functions.

This is a workaround for some quadratic complexity in ARCSequenceOpt which calls canApplyDecrementRefCount very frequently.

rdar://problem/56268570

Author: eeckstein

lib/SILOptimizer/Analysis/AliasAnalysis.cpp

@@ -661,6 +661,16 @@ AliasResult AliasAnalysis::aliasInner(SILValue V1, SILValue V2,
 }
 
 bool AliasAnalysis::canApplyDecrementRefCount(FullApplySite FAS, SILValue Ptr) {
+  // If the connection graph is invalid due to a very large function, we also
+  // skip all other tests, which might take significant time for a very large
+  // function.
+  // This is a workaround for some quadratic complexity in ARCSequenceOpt.
+  // TODO: remove this check once ARCSequenceOpt is retired or the quadratic
+  // behavior is fixed.
+  auto *conGraph = EA->getConnectionGraph(FAS.getFunction());
+  if (!conGraph->isValid())
+    return true;
+
   // Treat applications of no-return functions as decrementing ref counts. This
   // causes the apply to become a sink barrier for ref count increments.
   if (FAS.isCalleeNoReturn())