[sledge] Fix excessive existential sinking in Sh.simplify
[sledge] Fix excessive existential sinking in Sh.simplify

Summary:
Sh.simplify could incorrectly partition the scope of an existential if
it appeared only in multiple disjunctions but not anywhere above.

Differential Revision: D31531526

fbshipit-source-id: 1ecd2eb33

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