This allows you to see the output immediately after desugaring
but before any optimisation.

I've wanted this for some time, but I was triggered into action
by Trac #13032 comment:9.

Interestingly, the change means that with -dcore-lint we will
now Lint the output before the very simple optimiser;
and this showed up Trac #14749.  But that's not the fault
of -ddump-ds-preopt!