Selectively revert "SI-8315 Better debugging facility for ICode"

This reverts commit 0561dd084b5f3c2678eb032a40b85cb25bb6d589,
because appending the phase name to the icode filename breaks
the windows build. Only doing it under -Ydebug.
Merge pull request #3586 from adriaanm/windows-opt-fail-revert

Revert "SI-8315 Better debugging facility for ICode"