Make sure that exprIsTrivial responds "False" to
	(scc "foo" x)

We do not treat (_scc_ "foo" x) as trivial, because
  a) it really generates code, (and a heap object when it's
     a function arg) to capture the cost centre
  b) see the note [SCC-and-exprIsTrivial] in Simplify.simplLazyBind