traceTc and friends allocate even when tracing is disabled

While staring at Core from a typechecker module I noticed that traceTc currently allocates. Specifically, Ticky-Ticky revealed that we allocate hundreds of millions of SDoc thunks while typechecking Cabal's Distribution.SPDX.LicenseId module. For instance, in GHC.Tc.Utils.Zonk.commitFlexi (called from GHC.Tc.Utils.Zonk.zonkTyVarOcc) we allocated 24 million documents of each of the "Defaulting flexi tyvar to ..." documents. This is simply awful.

The problem is that we fail to inline the check for whether tracing is enabled or not. That is, we have:

trace :: SDoc -> IO ()
trace doc =
  if tracingIsEnabled
  then emitTraceMsg doc
  else return ()

somewhereElse =
  let thunk = ... :: SDoc
  in trace thunk >> ...

What we rather want is to inline trace and push the allocation into the conditional:

somewhereElse =
 (if traceIsEnabled
  then let thunk = ... :: SDoc
       in emitTraceMsg thunk
  else return ()) >> ...