[Relay] optimize dumpir time

[Yangzijun510]

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Add string before doc, slow. Add string after doc, fast

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[Yangzijun510]

I tested that
Doc doc; for (int i = 0; i <= 100000; i++) { Doc a; a << "a"; doc = a << doc; }; and Doc docc; for (int i = 0; i <= 100000; i++) { Doc a; a << "a"; docc = docc << a; } First cost time is 150s Second cost time is 1s

[Destinywck]

@Hzfengsy cc