[mlir] Use std::lcm (NFC)

This patch replaces mlir::lcm with std::lcm, a C++17 feature.

Note that all the arguments to mlir::lcm are of int64_t with no
implicit type conversion as they are passed to mlir::lcm, which I've
verified by modifying mlir::lcm as:

  template <typename TA, typename TB>
  inline int64_t lcm(TA a, TB b) {
    static_assert(std::is_same_v<TA, int64_t>);
    static_assert(std::is_same_v<TB, int64_t>);
    :