[Arith] Simplify floordiv(x*8+7, 16) to floordiv(x, 2)

[wrongtest-intellif]

Hi, the PR add a simplify rule to allow patterns like floordiv(x*8+7, 16) => floordiv(x, 2).

We find the issue when using compute_at to repeat op:

T_add = T.alloc_buffer([4], dtype="float32")
    for i0 in T.serial(4):
        with T.block("T_add"):
            ax0 = T.axis.spatial(4, i0)
            T_add[ax0] = x[ax0] + 1.0
    for i0_0, i0_1 in T.grid(8, 8):
        with T.block("T_repeat"):
            ax0 = T.axis.spatial(64, i0_0 * 8 + i0_1)
            T_repeat[ax0] = T_add[ax0 // 16]

compute_at(T_add , i0_0) currently will result to a "dynamic" extent floordiv(x*8 + 7, 16) - floordiv(x, 2) + 1.

Also fix a typo of previous compute at testcase.

[kparzysz-quic]

The math checks out. The rest looks ok as well.