simplify IfThenElse conditions with ISL

[Superjomn]

Add something like

• ConditionedSimplify to simplify Load/Store indices
• Extract extent from PolyFor to construct a For

  if ((blockIdx.x < 40)) {
  {
    if ((threadIdx.x < 40)) {
    {
      if (((((blockIdx.x >= 0) && (blockIdx.x <= 39)) && (threadIdx.x >= 0)) && (threadIdx.x <= 39))) {
        C[0] = A[((40 * blockIdx.x) + threadIdx.x)];
      };
      C_cache_write_out_3[((40 * blockIdx.x) + threadIdx.x)] = C[0];
    }
    };
  }
  };