fix a potential crash case for vertical merging
fix a potential crash case for vertical merging

Summary: Didn't set the don't merge stage for instruction's field class which might crash later after merge.

Reviewed By: Feng23

Differential Revision: D28005611

fbshipit-source-id: c9872c6b2885bad92427a240c245dde7e180751d

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex