A fine bug report (#3403) demonstrated that we were losing the tail
call property when a complicated pattern match was involved.  After
a bit of investigation I discovered that the culprit was the failure
join-point introduced by the pattern matcher.  It was a zero-argument
thunk, which is not very CPR-friendly, and that interacted badly with
CPR worker/wrapper.

It's easy to fix, the same way that we fix other join points, by supplying
a dummy argument (that is not really passed at runtime.