[llvm] [SLP][REVEC] Disable strided load if the source is vector instruction. (PR #99462)

[Alexey Bataev via llvm-commits]

================
@@ -4617,12 +4621,13 @@ BoUpSLP::LoadsState BoUpSLP::canVectorizeLoads(
       // 3. The loads are ordered, or number of unordered loads <=
       // MaxProfitableUnorderedLoads, or loads are in reversed order.
       // (this check is to avoid extra costs for very expensive shuffles).
-      if (IsPossibleStrided && (((Sz > MinProfitableStridedLoads ||
-                                  (static_cast<unsigned>(std::abs(*Diff)) <=
-                                       MaxProfitableLoadStride * Sz &&
-                                   isPowerOf2_32(std::abs(*Diff)))) &&
-                                 static_cast<unsigned>(std::abs(*Diff)) > Sz) ||
-                                *Diff == -(static_cast<int>(Sz) - 1))) {
+      if (IsSourceScalarInstruction && IsPossibleStrided &&
----------------
alexey-bataev wrote:

```suggestion
      bool IsPossibleStrided = IsSourceScalarInstruction && (*Diff % (Sz - 1) == 0);
      // Try to generate strided load node if:
      // 1. Target with strided load support is detected.
      // 2. The number of loads is greater than MinProfitableStridedLoads,
      // or the potential stride <= MaxProfitableLoadStride and the
      // potential stride is power-of-2 (to avoid perf regressions for the very
      // small number of loads) and max distance > number of loads, or potential
      // stride is -1.
      // 3. The loads are ordered, or number of unordered loads <=
      // MaxProfitableUnorderedLoads, or loads are in reversed order.
      // (this check is to avoid extra costs for very expensive shuffles).
      if (IsPossibleStrided &&
```

https://github.com/llvm/llvm-project/pull/99462