[RISCV] Const correct reference argument to isElementRotate. NFC

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -4613,7 +4613,7 @@ static bool isMaskedSlidePair(ArrayRef<int> Mask,
 
 // Exactly matches the semantics of a previously existing custom matcher
 // to allow migration to new matcher without changing output.
-static bool isElementRotate(std::array<std::pair<int, int>, 2> &SrcInfo,
+static bool isElementRotate(const std::array<std::pair<int, int>, 2> &SrcInfo,
                             unsigned NumElts) {
   if (SrcInfo[1].first == -1)
     return true;