Only log the visit of a return instruction if we in fact found a return

instruction.

This avoids dereferencing null in the debug logging if the instruction
was not in fact a return instruction. This potential bug was found by
PVS-Studio.

This actually fixes the last of the "dereferenced a pointer before
checking it for null" reports in the recent PVS-Studio run. However,
there are quite a few reports of this nature that I did not do anything
to fix because they are pretty glaring false positives. They usually
took the form of quite clear correlated checks or a check made in
a separate function. I've even added asserts anywhere this correlation
wasn't pretty obvious and fundamental to the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285988 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/ObjCARC/ObjCARCOpts.cpp

@@ -2086,12 +2086,11 @@ void ObjCARCOpt::OptimizeReturns(Function &F) {
   SmallPtrSet<const BasicBlock *, 4> Visited;
   for (BasicBlock &BB: F) {
     ReturnInst *Ret = dyn_cast<ReturnInst>(&BB.back());
-
-    DEBUG(dbgs() << "Visiting: " << *Ret << "\n");
-
     if (!Ret)
       continue;
 
+    DEBUG(dbgs() << "Visiting: " << *Ret << "\n");
+
     const Value *Arg = GetRCIdentityRoot(Ret->getOperand(0));
 
     // Look for an ``autorelease'' instruction that is a predecessor of Ret and