This is very much a corner case, but Trac #15163 showed
that if you have a RULE like
    forall x. f (g x) = ..x..

and g = undefined, then the simplifier is likely to discard
that 'x' argument. It is usually right to do so; but not here
because then x is used on the right but not bound on the left.

The fix is a narrow one, aimed at this rather pathalogical case.
See Note [Do not expose strictness if sm_inline=False] in
SimplUtils.