[RISCV] Fix -Wsign-compare warning from f8ee58a

lib/Target/RISCV/RISCVISelLowering.cpp:4629:26: error: comparison of integers of different signs: 'unsigned int' and 'int' [-Werror,-Wsign-compare]
 4629 |   for (unsigned i = 0; i != NumElts; ++i) {
      |                        ~ ^  ~~~~~~~
1 error generated.

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -4626,7 +4626,7 @@ static bool isElementRotate(const std::array<std::pair<int, int>, 2> &SrcInfo,
 static bool isAlternating(const std::array<std::pair<int, int>, 2> &SrcInfo,
                           ArrayRef<int> Mask, bool RequiredPolarity) {
   int NumElts = Mask.size();
-  for (unsigned i = 0; i != NumElts; ++i) {
+  for (int i = 0; i != NumElts; ++i) {
     int M = Mask[i];
     if (M < 0)
       continue;