v8.4: A few improvements on pattern-matching compilation which fix

- Optimize the removal of generalization when there is no dependency in
  the generalized variable (see postprocess_dependencies, and the removal
  of dependencies in the default type of impossible cases). This fixes
  #3733 (though with a little risk of incompatibilities).
- Compute the onlydflt flag correctly (what allows automatic treatment
  of impossible cases even when there is no clause at all).
- Added support for several impossible cases in compilation of "match".
  Altogether, this fixes #3388.

(backport of 2 commits from trunk)