Fix potential invalid reads from `++`.
Fix potential invalid reads from `++`.

I suspect that the compiler is causing this by helpfully doing reads of op
and tree type together since they're next to each other. Unfortunately, op
wasn't set except for binary ops. This now sets op each time, just to make
sure these invalid reads go away.

FascinatedBox/lily,jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily,jesserayadkins/lily,jesserayadkins/lily,FascinatedBox/lily