[sledge] Remove context-preservation assumption in Sh.simplify
[sledge] Remove context-preservation assumption in Sh.simplify

Summary:
The implementation of Sh.simplify assumes that starjoining the stem
and disjunctions back together does not add variables to the
context. This will not be true if singleton disjunctions are
simplified away. This diff removes this assumption.

Differential Revision: D31283451

fbshipit-source-id: 8be57158a

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