[mlir] Fix -Wsign-compare in AffineOps.cpp (NFC)

/llvm-project/mlir/lib/Dialect/Affine/IR/AffineOps.cpp:4590:25:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
  if (dynamicBasisIndex == dynamicBasis.size())
      ~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~
1 error generated.

Author: DamonFool

mlir/lib/Dialect/Affine/IR/AffineOps.cpp

@@ -4577,7 +4577,7 @@ static std::optional<SmallVector<int64_t>>
 foldCstValueToCstAttrBasis(ArrayRef<OpFoldResult> mixedBasis,
                            MutableOperandRange mutableDynamicBasis,
                            ArrayRef<Attribute> dynamicBasis) {
-  int64_t dynamicBasisIndex = 0;
+  uint64_t dynamicBasisIndex = 0;
   for (OpFoldResult basis : dynamicBasis) {
     if (basis) {
       mutableDynamicBasis.erase(dynamicBasisIndex);