[SPARK-5454] More robust handling of self joins

[marmbrus]

Also I fix a bunch of bad output in test cases.

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Test build #27270 has started for PR 4520 at commit 55d64b3.

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Test build #27271 has started for PR 4520 at commit 6fc38de.

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Test build #27271 has finished for PR 4520 at commit 6fc38de.

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Test build #27276 has started for PR 4520 at commit 49c8e26.

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Test build #27276 has finished for PR 4520 at commit 49c8e26.

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[chenghao-intel]

space before =>

[chenghao-intel]

LGTM in general except some of the minor issues.

My original thought on this, is adding a new Project on top of the MultiInstanceRelation(if it appears more than once in the query tree), so we can still keep the same reference to the original MultiInstanceRelation instance, and this probably give us an opportunity to optimize the query by only computing the MultiInstanceRelation once.

Anyway, let's leave it for the future improvement.

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Test build #27296 has started for PR 4520 at commit 4f4a85c.

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Test build #27296 has finished for PR 4520 at commit 4f4a85c.

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