[llvm] [RISCV] Refactor performCONCAT_VECTORSCombine. NFC (PR #69068)

[Michael Maitland via llvm-commits]

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@@ -13867,26 +13844,25 @@ static SDValue performCONCAT_VECTORSCombine(SDNode *N, SelectionDAG &DAG,
   if (!TLI.isLegalStridedLoadStore(WideVecVT, Align))
     return SDValue();
 
+  auto [Stride, Reversed] = *BaseDiff;
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michaelmaitland wrote:

Looking at #68726, I wonder if `Reversed` may not be the best name for this variable? The RVV spec uses the term `negative stride`. Additionally, when `BaseIndexOffset` returns a negative value, it does not need to be negated here although that load is "in reverse". I suggest we rename this to something along the lines of `MustNegateStride`?

https://github.com/llvm/llvm-project/pull/69068