Remove the `isInlinePragma prag` test

This is a spin off of ticket:11781#comment:118828, where simonpj writes:

What is more mysterious to me is why the unfolding for $fApplicativeIO_$c*> doesn't have bindIO, and then bindIO inlined into it, which would allow a cascade of further improvements, which would, I think, produce essentially the same code as thenIO.

The offending corner is this

active_unfolding_gentle id
=  isInlinePragma prag    -- WHY??
&& isEarlyActive (inlinePragmaActivation prag)

So I tried removing the isInlinePragma prag test, and indeed the code becomes identical.

Conclusion:

• I'd love to make the above change to active_unfolding_gentle and see if any other performance numbers budge. I doubt that anything will change a lot -- it really only affects whether optimisation happens before or after inlining -- but it should improve compiler performance a bit for that very reason. This could be a separate ticket

Incidentally, see #5928 which is somewhat related.

This ticket tracks that idea.