[llvm] [LV] Use getFixedValue instead of getKnownMinValue when appropriate (PR #143526)

[Luke Lau via llvm-commits]

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@@ -5342,6 +5342,10 @@ LoopVectorizationCostModel::getUniformMemOpCost(Instruction *I,
   StoreInst *SI = cast<StoreInst>(I);
 
   bool IsLoopInvariantStoreValue = Legal->isInvariant(SI->getValueOperand());
+  // TODO: We have tests that request the cost of extracting element
+  // VF.getKnownMinValue() - 1 from a scalable vector. This is actually
+  // meaningless, given what we actually want is the last lane and is likely
+  // to be more expensive.
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lukel97 wrote:

I see, FWIW `VPLane::Kind` has something similar to represent the last lane:

```c++
  /// Kind describes how to interpret Lane.
  enum class Kind : uint8_t {
    /// For First, Lane is the index into the first N elements of a
    /// fixed-vector <N x <ElTy>> or a scalable vector <vscale x N x <ElTy>>.
    First,
    /// For ScalableLast, Lane is the offset from the start of the last
    /// N-element subvector in a scalable vector <vscale x N x <ElTy>>. For
    /// example, a Lane of 0 corresponds to lane `(vscale - 1) * N`, a Lane of
    /// 1 corresponds to `((vscale - 1) * N) + 1`, etc.
    ScalableLast
  };
```

https://github.com/llvm/llvm-project/pull/143526