avoid redundant traversal in assert_ground_id
avoid redundant traversal in assert_ground_id

Summary:
We were going through the passed tvar's `lower` *and* `lowertvars`.
Now just `lower`.

Reviewed By: samwgoldman

Differential Revision: D3762139

fbshipit-source-id: ad5b788a6a62b5cba04568f75f17ecb7bfaddacd

AgentME/flow,mroch/flow,popham/flow,fletcherw/flow,chenglou/flow,jeffmo/flow,gabro/flow,chenglou/flow,fletcherw/flow,gabelevi/flow,claudiopro/flow,samwgoldman/flow,jgrund/flow,JonathanUsername/flow,samwgoldman/flow,ForbesLindesay/flow,JonathanUsername/flow,ForbesLindesay/flow,jgrund/flow,mroch/flow,fletcherw/flow,facebook/flow,jamesgpearce/flow,popham/flow,mroch/flow,nmote/flow,mroch/flow,ylu1317/flow,facebook/flow,gabro/flow,MichaelDeBoey/flow,AprilArcus/flow,chenglou/flow,ForbesLindesay/flow,claudiopro/flow,popham/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow,MichaelDeBoey/flow,AprilArcus/flow,mroch/flow,facebook/flow,ylu1317/flow,AprilArcus/flow,JonathanUsername/flow,popham/flow,jamesgpearce/flow,AgentME/flow,TiddoLangerak/flow,TiddoLangerak/flow,nmote/flow,fletcherw/flow,jamesgpearce/flow,gabelevi/flow,gabro/flow,JonathanUsername/flow,jeffmo/flow,claudiopro/flow,AgentME/flow,ForbesLindesay/flow,gabelevi/flow,jamesgpearce/flow,samwgoldman/flow,TiddoLangerak/flow,nmote/flow,fletcherw/flow,chenglou/flow,nmote/flow,ylu1317/flow,ylu1317/flow,jamesgpearce/flow,jgrund/flow,TiddoLangerak/flow,JonathanUsername/flow,jamesgpearce/flow,chenglou/flow,gabelevi/flow,facebook/flow,MichaelDeBoey/flow,AprilArcus/flow,mroch/flow,fletcherw/flow,jeffmo/flow,facebook/flow,claudiopro/flow,gabelevi/flow,ForbesLindesay/flow,JonathanUsername/flow,MichaelDeBoey/flow,samwgoldman/flow,nmote/flow,samwgoldman/flow,jgrund/flow,popham/flow,TiddoLangerak/flow,gabro/flow,AprilArcus/flow,mroch/flow,popham/flow,chenglou/flow,AprilArcus/flow,claudiopro/flow,AgentME/flow,jeffmo/flow,facebook/flow,jgrund/flow,facebook/flow,gabro/flow,JonathanUsername/flow,jamesgpearce/flow,jgrund/flow,popham/flow,ylu1317/flow,jeffmo/flow,jeffmo/flow,samwgoldman/flow,ForbesLindesay/flow,AgentME/flow,nmote/flow,MichaelDeBoey/flow,nmote/flow,gabro/flow