[llvm] [RISCV] Use slideup to lower build_vector when its last operand is an extraction (PR #154450)

[Luke Lau via llvm-commits]

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@@ -4512,42 +4512,97 @@ static SDValue lowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG,
          "Illegal type which will result in reserved encoding");
 
   const unsigned Policy = RISCVVType::TAIL_AGNOSTIC | RISCVVType::MASK_AGNOSTIC;
+  auto getVSlide = [&](bool SlideUp, EVT ContainerVT, SDValue Passthru,
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lukel97 wrote:

You could also move the definition down below `SlideUp` and then capture that instead of having it as an argument. I think Mask + VL can also be captured too, since you're already capturing Policy?

https://github.com/llvm/llvm-project/pull/154450