Summary:
Previously, the logic that checks whether a thunk has a counter or not
was duplicated in multiple functions.

This led to thunk enters being accounted to their enclosing functions in
`StgCmmTicky.tickyEnterThunk`, because the outer call to
`withNewTickyCounterThunk` didn't set the counter label for the thunk.
And rightly so! `tickyEnterThunk` should only account thunk enters to a
counter if `-ticky-dyn-thunk` is on.

This patch extracts the logic that was already present in its most
general form in `withNewTickyCounterThunk` into its own functions and
lets all other call sites checking for `-ticky-dyn-thunk` call this new
function named `thunkHasCounter` instead.

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5392