MachineModel: Inconsequential TableGen SubtargetEmitter fix.

Drive by fix. I noticed some missing logic that might bite future
users. This shouldn't affect the final output on currently modeled
targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174142 91177308-0d34-0410-b5e6-96231b3b80d8