Minor optimization to avoid ADLB operation when zero refcount change in case generated code does this.
Minor optimization to avoid ADLB operation when zero refcount change in case generated code does this.

git-svn-id: 51d371801c0de2a0625fbca80cd99d181c32913f@6342 dc4e9af1-7f46-4ead-bba6-71afc04862de

JohnPJenkins/swift-t,basheersubei/swift-t,basheersubei/swift-t,blue42u/swift-t,basheersubei/swift-t,blue42u/swift-t,JohnPJenkins/swift-t,swift-lang/swift-t,basheersubei/swift-t,blue42u/swift-t,JohnPJenkins/swift-t,JohnPJenkins/swift-t,JohnPJenkins/swift-t,swift-lang/swift-t,basheersubei/swift-t,swift-lang/swift-t,blue42u/swift-t,blue42u/swift-t,blue42u/swift-t,blue42u/swift-t,basheersubei/swift-t,JohnPJenkins/swift-t,swift-lang/swift-t,JohnPJenkins/swift-t,swift-lang/swift-t,swift-lang/swift-t,swift-lang/swift-t,basheersubei/swift-t