Revert pattern matching improvements introducing regressions in 3.3.4

[WojciechMazur]

This change reverts pattern-matching improvements backported to 3.3.4 but causes new regressions.
The goal is to unblock the 3.3.4 release in case a proper fix to #21569 would not be possible to achive in a short period of time.

Reverts fully #21549 (with exception of i21218 test case reproducer) to mitigate regressions of #21569
Reverts single (major) commit from #20669 - 333fa6c and adapts tests to mitigate regression introduced in #21218

[dwijnand]

Do you want to add tests/warn/i21218.scala from PR #21532 ?

[WojciechMazur]

Do you want to add tests/warn/i21218.scala from PR #21532 ?

This one is already present in the release-3.3.4 branch. It was backported in #21549, it's the only not reverted commit from that PR to ensure the regression is fixed