[sledge] Fix: Sh.pure
[sledge] Fix: Sh.pure

Summary:
It was incorrect in case any but the first of of the Formula.disjuncts
was inconsistent.

Reviewed By: ngorogiannis

Differential Revision: D23459519

fbshipit-source-id: 394677e38

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