[llvm] [DAGCombiner] cache negative result from getMergeStoreCandidates() (PR #106949)

[Matt Arsenault via llvm-commits]

================
@@ -191,6 +191,11 @@ namespace {
     // AA - Used for DAG load/store alias analysis.
     AliasAnalysis *AA;
 
+    /// This caches all chains that have already been processed in
+    /// DAGCombiner::getStoreMergeCandidates() and found to have no mergeable
+    /// stores candidates.
+    SmallSet<SDNode *, 32> ChainsWithoutMergeableStores;
----------------
arsenm wrote:

I didn't know it had the specialization, last I looked it didn't. I guess I still expect to see SmallPtrSet. These also are supposed to have a default value, so you should just use that unless you have a specific reason to pick another number 

https://github.com/llvm/llvm-project/pull/106949