Fix a bug in CSE, for INLINE/INLNEABLE things
Previusly we simply weren't doing CSE at all on things whose unfolding were not always-active, for reasons explained in Note [CSE for INLINE and NOINLINE]. But that was bad! Making something INLNEABLE meant that its RHS was no longer CSE'd, and that made some nofib programs worse. And it's entirely unnecessary. I thoguht it through again, wrote new comments (under the same Note), and things are better again.