When debugging Trac #14650, I found a place where we had

    let {-# INLINEABLE f #-}
        f = BIG
    in f 7

but 'f' wasn't getting inlined at its unique call site.
There's a good reason for that with INLINE things, which
should only inline when saturated, but not  for INILNEABLE
things.

This patch narrows the case where preInlineUnconditionally
gives up.  It significantly shortens (and improves) the code
for #14650.