Improve SpecConstr (esp nofib/spectral/ansi)

This MR makes three improvements to SpecConstr: see #24282

* It fixes an outright (and recently-introduced) bug in `betterPat`, which
  was wrongly forgetting to compare the lengths of the argument lists.

* It enhances ConVal to inclue a boolean for work-free-ness, so that the
  envt can contain non-work-free constructor applications, so that we
  can do more: see Note [ConVal work-free-ness]

* It rejigs `subsumePats` so that it doesn't reverse the list.  This can
  make a difference because, when patterns overlap, we arbitrarily pick
  the first.  There is no "right" way, but this retains the old
  pre-subsumePats behaviour, thereby "fixing" the regression in #24282.

Nofib results

   +========================================
   |                 spectral/ansi  -21.14%
   | spectral/hartel/comp_lab_zift   -0.12%
   |       spectral/hartel/parstof   +0.09%
   |           spectral/last-piece   -2.32%
   |           spectral/multiplier   +6.03%
   |                 spectral/para   +0.60%
   |               spectral/simple   -0.26%
   +========================================
   |                     geom mean   -0.18%
   +----------------------------------------

The regression in `multiplier` is sad, but it simply replicates GHC's
previous behaviour (e.g. GHC 9.6).