Return checking is now done as a general flow check at the end
of function compilation, since break and goto complicated the
way I was doing return checking before.  Goto-over-declaration
checking is also done as a final flow check.

Temporary variables used for effect extraction are now
actually temporary.  Otherwise "op=", "++", and "--" appear as
declarations that cannot be jumped over.

R=rsc
APPROVED=rsc
DELTA=421  (344 added, 38 deleted, 39 changed)
OCL=32527
CL=32535