[purity] Use instruction index for gettting inferbo result
[purity] Use instruction index for gettting inferbo result

Reviewed By: ezgicicek

Differential Revision: D27735553

fbshipit-source-id: cb27c2f9b

facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer