speed up scope node processing: if the first element of a scope

entry we're about to process is obviously going to fail, don't
bother pushing a scope only to have it immediately be popped.
This avoids a lot of scope stack traffic in common cases.

Unfortunately, this requires duplicating some of the predicate
dispatch.  To avoid duplicating the actual logic I pulled each
predicate out to its own static function which gets used in
both places.

llvm-svn: 97651