Fix longstanding bug in compilation of predicates. Expressions like

Fix longstanding bug in compilation of predicates.  Expressions like
this

  (if (if (bar numerator)
	  (foo denominator)
	  (not (foo denominator)))
      numerator
      denominator)

would compile incorrectly by making both calls to `foo' have the same
continuation.  This should have been noticed as an error because the
continuation entry hooks for that continuation were attached to both
expressions: when they were combined with the call to `bar' the hooks
were used to attach the continuation to the enclosing expression --
but each side of the test was attached twice.  Error checking in the
node-attachment code would have caught this one at compile time.

The new code generates a new continuation for the call to `foo' whose
result is inverted, and then codes the `not' using a PCFG and two
constants -- all of which gets folded out later, resulting in the
desired code.