[llvm] [LoopFusion][NFC] Avoid copying fusion candidates per pair (PR #203461)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Madhur Amilkanthwar (madhur13490)

<details>
<summary>Changes</summary>

`fuseCandidates()` copied both candidates (each holding two `SmallVector<Instruction *, 16>`) for every adjacent pair examined, even pairs rejected by an early continue. Bind them by const reference; they are only read before being erased from the list, and performFusion runs before the erases.

---
Full diff: https://github.com/llvm/llvm-project/pull/203461.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/LoopFuse.cpp (+2-2) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/LoopFuse.cpp b/llvm/lib/Transforms/Scalar/LoopFuse.cpp
index b5126f9f3b704..1b83c971c01bf 100644
--- a/llvm/lib/Transforms/Scalar/LoopFuse.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopFuse.cpp
@@ -746,8 +746,8 @@ struct LoopFuser {
       for (auto It = CandidateList.begin(), NextIt = std::next(It);
            NextIt != CandidateList.end(); It = NextIt, NextIt = std::next(It)) {
 
-        auto FC0 = *It;
-        auto FC1 = *NextIt;
+        const FusionCandidate &FC0 = *It;
+        const FusionCandidate &FC1 = *NextIt;
 
         assert(!LDT.isRemovedLoop(FC0.L) &&
                "Should not have removed loops in CandidateList!");

``````````

</details>


https://github.com/llvm/llvm-project/pull/203461