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

[Min-Yih Hsu 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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mshockwave wrote:

> Nit, probably better to use an explicit captures list

There is also `DL`, and I decided to capture `SlideUp` so the list becomes a bit long and I prefer using the default capture.

> I think Mask + VL can also be captured too, 

Ah, I was going to do that originally but since Mask and VL were created from `auto [Mask, VL] = ...` they're considered being _bindings_ rather than variables and you cannot capture non-variables.

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