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

[Princeton Ferro via llvm-commits]

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@@ -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;
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Prince781 wrote:

> I didn't know it had the specialization, last I looked it didn't.

https://github.com/llvm/llvm-project/blob/af1e59aea2ea7d07ece1f34621dda38c995826a3/llvm/include/llvm/ADT/SmallSet.h#L253-L256

But I see this isn't used too frequently in the compiler, so I'll switch to `SmallPtrSet`.

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