[flang] Fix compilation error on macOS

The call to `std::min` failed to compile with GCC on macOS due to type
inference because `std::size_t` is `long unsigned int` but `std::int64_t`
is `long long int`.

Original-commit: flang-compiler/f18@c342575a9e0217fa3252b70b8ef3eac0236a58e3
Reviewed-on: https://github.com/flang-compiler/f18/pull/977