fix bug for negative shift value
fix bug for negative shift value

Reviewed By: luciang

Differential Revision: D3407128

fbshipit-source-id: 2c00387d6e068f9e052f539198ae2f985d265c74

floxard/folly,mqeizi/folly,facebook/folly,Orvid/folly,floxard/folly,rklabs/folly,rklabs/folly,charsyam/folly,rklabs/folly,facebook/folly,facebook/folly,mqeizi/folly,rklabs/folly,mqeizi/folly,floxard/folly,brunomorishita/folly,brunomorishita/folly,charsyam/folly,brunomorishita/folly,charsyam/folly,facebook/folly,brunomorishita/folly,Orvid/folly,floxard/folly,mqeizi/folly,Orvid/folly,Orvid/folly,Orvid/folly,brunomorishita/folly,facebook/folly,mqeizi/folly,rklabs/folly,charsyam/folly,floxard/folly