GHC.Cmm.Opt: Be stricter in results.

Optimization either returns Nothing if nothing is to be done or
`Just <cmmExpr>` otherwise. There is no point in being lazy in
`cmmExpr`. We usually inspect this element so the thunk gets forced
not long after.

We might eliminate it as dead code once in a blue moon but that's
not a case worth optimizing for.

Overall the impact of this is rather low. As Cmm.Opt doesn't allocate
much (compared to the rest of GHC) to begin with.