[sledge] Lazily allocate hashtables for unique void names
[sledge] Lazily allocate hashtables for unique void names

Summary:
Only void-returning function calls need a table, other instructions
just take the next SSA name. So avoid allocate tables in the second
case by creating them lazily.

Reviewed By: bennostein

Differential Revision: D38540965

fbshipit-source-id: 12b234696510e85e211c11e21f12594415a997bf

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