[llvm] [RISCV] Handle fixed length vectors with exact VLEN in lowerINSERT_SUBVECTOR (PR #84107)

[Yeting Kuo via llvm-commits]

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@@ -2063,8 +2063,16 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
     MVT SubVecContainerVT = SubVecVT;
     // Establish the correct scalable-vector types for any fixed-length type.
     if (SubVecVT.isFixedLengthVector()) {
-      assert(Idx == 0 && V.isUndef());
       SubVecContainerVT = TLI.getContainerForFixedLengthVector(SubVecVT);
+#ifndef NDEBUG
+      TypeSize VecRegSize = TypeSize::getScalable(RISCV::RVVBitsPerBlock);
+      bool ExactlyVecRegSized =
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yetingk wrote:

need to add `[[maybe_unused]]` to suppress unused variable warning.

https://github.com/llvm/llvm-project/pull/84107