improvements to ShortenSrcStringsPass
improvements to ShortenSrcStringsPass

Summary:
This increases the effectiveness of the ShortenSrcStringsPass by
- moving it to the end of the pass list so that it won't use strings that dead-code elimination will kill afterwards
- preferring strings that will get low indices

Not sure if any of the source_file data is actually relevant in the end; but those strings ARE generally kept alive via gather_strings.

Reviewed By: int3

Differential Revision: D14633963

fbshipit-source-id: 0e95486daaf6257cb355bc8028d5de444e673ba2

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