SI-7427 stop crashing under -Ydebug.

Change "dumpClassesAndAbort" to "devWarning". You can witness
it happen like so.

  % scalac test/files/pos/t7427.scala -Ydebug -Xdev
  ...
  [running phase cleanup on t7427.scala]
  [running phase icode on t7427.scala]
  warning: !!! PJUMP(method matchEnd4)/scala.tools.nsc.backend.icode.GenICode$PJUMP is not a control flow instruction
  warning: !!! PJUMP(method case6)/scala.tools.nsc.backend.icode.GenICode$PJUMP is not a control flow instruction
  [running phase inliner on t7427.scala]
  [running phase inlinehandlers on t7427.scala]

Having now lived with this for months, I have no ambition to
unravel the actual problem, I just want it to stop crashing.
Merge pull request #2565 from paulp/issue/7427

SI-7427 stop crashing under -Ydebug.