When inlining, we are making a copy of the expression, so we have to
be careful about duplicating work.  Previously we were using
exprIsCheap for that, but it is willing to duplicate a cheap primop --
and that is terribly bad if it happens inside some inner array loop
(Trac #5623).  So now we use a new function exprIsWorkFree.  Even
then there is some wiggle room:
   see Note [exprIsWorkFree] in CoreUtils

This commit does make wheel-sieve1 allocate a lot more, but we decided
that's just tough; it's more important for inlining to be robust
about not duplicating work.