[sledge] Fix existential mishandling in Sh.simplify
[sledge] Fix existential mishandling in Sh.simplify

Summary:
It was possible for the scope of existentials to be violated. In
particular, before this diff the order of re-quantifying the
existentials and conjoining the non-eliminated equations from the
solution of solving for existentials was wrong.

Reviewed By: ngorogiannis

Differential Revision: D24746231

fbshipit-source-id: d96cc60a6

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