[TIR] Fix dtype mismatch error due to LetStmt

[masahi]

Previously, even if the dtype of the rhs of LetStmt is casted to int64, the bound variable doesn't get updated. This results in the following invalid IR to be generated:

p0_red_temp_v0 = T.alloc_buffer([T.int64(1), T.int64(56), T.int64(56)], dtype="int32")
...
for ax0, ax1, ax2, k3 in T.grid(T.int64(1), T.int64(56), T.int64(56), T.int64(64)):
   with T.block("p0_red_temp"):
        ...
        v_p0_red_temp_v0: T.int32 = T.Select(p0_red_temp_v1[v_ax0, v_ax1, v_ax2] > p0[v_ax0, v_ax1, v_ax2, v_k3] or (p0_red_temp_v1[v_ax0, v_ax1, v_ax2] == p0[v_ax0, v_ax1, v_ax2, v_k3] and T.Cast("int64", p0_red_temp_v0[v_ax0, v_ax1, v_ax2]) < v_k3), T.Cast("int64", p0_red_temp_v0[v_ax0, v_ax1, v_ax2]), v_k3)
        p0_red_temp_v0[v_ax0, v_ax1, v_ax2] = v_p0_red_temp_v0

cc @vinx13

[tvm-bot]

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• cc @Hzfengsy, @junrushao, @quic-sanirudh _{See #10317 for details}

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