Avoid overly large SmallPtrSet/SmallSet

These sets perform linear searching in small mode so it is never a good
idea to use SmallSize/N bigger than 32.

llvm-svn: 259283
diff --git a/llvm/lib/Transforms/IPO/MergeFunctions.cpp b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
index 8a209a1..702aaa9 100644
--- a/llvm/lib/Transforms/IPO/MergeFunctions.cpp
+++ b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
@@ -1249,7 +1249,7 @@
   // functions, then takes each block from each terminator in order. As an
   // artifact, this also means that unreachable blocks are ignored.
   SmallVector<const BasicBlock *, 8> FnLBBs, FnRBBs;
-  SmallSet<const BasicBlock *, 128> VisitedBBs; // in terms of F1.
+  SmallPtrSet<const BasicBlock *, 32> VisitedBBs; // in terms of F1.
 
   FnLBBs.push_back(&FnL->getEntryBlock());
   FnRBBs.push_back(&FnR->getEntryBlock());