Teach SF to pull LETs and BEGINs out of IF predicates.

Teach SF to pull LETs and BEGINs out of IF predicates.

LIAR doesn't do a very good job with disjunctions not immediately in
the predicate position of IFs.  It still doesn't do a very good job
with (OR X (LET ((Y ...)) (OR ...))), but this helps it a little to
reduce needless pushing and popping of #F on the stack.