[llvm] [SLP] Use named structs in vectorizeStores() (NFC) (PR #132781)

[Gaëtan Bossu via llvm-commits]

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@@ -19923,31 +19923,51 @@ bool SLPVectorizerPass::vectorizeStores(
   BoUpSLP::ValueSet VectorizedStores;
   bool Changed = false;
 
+  /// A store instruction and the distance of its address to a base pointer.
+  struct CandidateInstr {
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gbossu wrote:

Fine by me.

Note for the future: Just wondering what you'd think about actually turning `using CandidateSet = std::set<CandidateInstr, StoreDistCompare>;` into a `std::map<int, unsigned> DistToInstIdx>`? I.e. using the distance as the map key. This is essentially how we are using the `std::set` already, but we need a special comparator function and a struct to make the set look like a map.

https://github.com/llvm/llvm-project/pull/132781