Pmc: Better SCC annotations and trace output

While investigating #20106, I made a few refactorings to the pattern-match
checker that I don't want to lose. Here are the changes:

  * Some key functions of the checker now have SCC annotations
  * Better `-ddump-ec-trace` diagnostics for easier debugging. I added
    'traceWhenFailPm' to see *why* a particular `MaybeT` computation fails and
    made use of it in `instCon`.

I also increased the acceptance threshold of T11545, which seems to fail
randomly lately due to ghc/max flukes.