See Note [ABot branches: use max] in CoreArity.

I stumbled on this when investigating something else, and
opened Trac #13031 to track it.

It's very hard to tickle the bug, which is why it has lurked so long,
but the test
   stranal/should_compile/T13031
does so

Oddly, the testsuite framework doesn't actually run the test; I have
no idea why.