[RISCV] Fix warnings

This patch fixes:

  llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5723:24: error: captured
  structured bindings are a C++20 extension
  [-Werror,-Wc++20-extensions]

  llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5728:76: error: captured
  structured bindings are a C++20 extension
  [-Werror,-Wc++20-extensions]

Author: kazutakahirata

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -5473,7 +5473,11 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
 
   MVT ContainerVT = getContainerForFixedLengthVector(DAG, VT, Subtarget);
 
-  auto [TrueMask, VL] = getDefaultVLOps(VT, ContainerVT, DL, DAG, Subtarget);
+  // Store the return value in a single variable instead of structured bindings
+  // so that we can pass it to GetSlide below, which cannot capture structured
+  // bindings until C++20.
+  auto TrueMaskVL = getDefaultVLOps(VT, ContainerVT, DL, DAG, Subtarget);
+  auto [TrueMask, VL] = TrueMaskVL;
 
   if (SVN->isSplat()) {
     const int Lane = SVN->getSplatIndex();
@@ -5716,6 +5720,7 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
     };
     auto GetSlide = [&](const std::pair<int, int> &Src, SDValue Mask,
                         SDValue Passthru) {
+      auto [TrueMask, VL] = TrueMaskVL;
       SDValue SrcV = GetSourceFor(Src);
       int SlideAmt = Src.second;
       if (SlideAmt == 0) {