Only use MASK-SIGNED-FIELD VOPs as last resorts

 The MOVE hack usually leads to better code when it can be used.
Insert error traps after full calls inferred not to return

 An explicit error trap after full calls to known functions helps
 understand type derivation errors at runtime; it's certainly better
 than executing arbitrary bytes.

 Only do this when the return type was tightened to NIL via type
 derivation; if a function is defknowned not to return, it really
 shouldn't.