Fix bug in the open-coder for HEAP-AVAILABLE? with type checking

Fix bug in the open-coder for HEAP-AVAILABLE? with type checking
turned on.  It was passing the FINISH continuation on to
OPEN-CODE:WITH-CHECKS, but that would call FINISH with an RTL
expression, whereas the callers of the open-coder want an RTL
predicate CFG.  Now, like other predicate open-coders,
HEAP-AVAILABLE?'s wraps the expression in a true test.