Resolve '-Wsign-compare' issue in 'gcc/omp-low.cc:lower_rec_simd_input_clauses'

Resolve '-Wsign-compare' issue in 'gcc/omp-low.cc:lower_rec_simd_input_clauses'

..., introduced in og12 commit 55722a87dd223149dcd41ca9c8eba16ad5b3eddc
"openmp: fix max_vf setting for amdgcn offloading":

    In file included from [...]/source-gcc/gcc/coretypes.h:482,
                     from [...]/source-gcc/gcc/omp-low.cc:27:
    [...]/source-gcc/gcc/poly-int.h: In instantiation of ‘typename if_nonpoly<Ca, bool>::type maybe_lt(const Ca&, const poly_int_pod<N, Cb>&) [with unsigned int N = 1; Ca = int; Cb = long unsigned int; typename if_nonpoly<Ca, bool>::type = bool]’:
    [...]/source-gcc/gcc/poly-int.h:1510:7:   required from ‘poly_int<N, typename poly_result<Ca, typename if_nonpoly<Cb>::type>::type> ordered_max(const poly_int_pod<N, C>&, const Cb&) [with unsigned int N = 1; Ca = long unsigned int; Cb = int; typename poly_result<Ca, typename if_nonpoly<Cb>::type>::type = long unsigned int; typename if_nonpoly<Cb>::type = int]’
    [...]/source-gcc/gcc/omp-low.cc:5180:33:   required from here
    [...]/source-gcc/gcc/poly-int.h:1384:12: error: comparison of integer expressions of different signedness: ‘const int’ and ‘const long unsigned int’ [-Werror=sign-compare]
     1384 |   return a < b.coeffs[0];
          |          ~~^~~~~~~~~~~
    [...]/source-gcc/gcc/poly-int.h: In instantiation of ‘typename if_nonpoly<Cb, bool>::type maybe_lt(const poly_int_pod<N, C>&, const Cb&) [with unsigned int N = 1; Ca = long unsigned int; Cb = int; typename if_nonpoly<Cb, bool>::type = bool]’:
    [...]/source-gcc/gcc/poly-int.h:1515:2:   required from ‘poly_int<N, typename poly_result<Ca, typename if_nonpoly<Cb>::type>::type> ordered_max(const poly_int_pod<N, C>&, const Cb&) [with unsigned int N = 1; Ca = long unsigned int; Cb = int; typename poly_result<Ca, typename if_nonpoly<Cb>::type>::type = long unsigned int; typename if_nonpoly<Cb>::type = int]’
    [...]/source-gcc/gcc/omp-low.cc:5180:33:   required from here
    [...]/source-gcc/gcc/poly-int.h:1373:22: error: comparison of integer expressions of different signedness: ‘const long unsigned int’ and ‘const int’ [-Werror=sign-compare]
     1373 |   return a.coeffs[0] < b;
          |          ~~~~~~~~~~~~^~~

	gcc/
	* omp-low.cc (lower_rec_simd_input_clauses): For 'ordered_max',
	cast 'omp_max_simt_vf ()', 'omp_max_simd_vf ()' to 'unsigned'.