[sledge] Simplify and (minor) strengthen Context.is_valid_eq
[sledge] Simplify and (minor) strengthen Context.is_valid_eq

Summary:
Use Theory.solvable_trms for a minor strengthening and simplification
of Context.is_valid_eq.

Reviewed By: jvillard

Differential Revision: D25883735

fbshipit-source-id: 3d0f3fcfe

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