It is fairly common to call SE->getConstant(Ty, 0) or
SE->getConstant(Ty, 1); this change makes such uses a little bit
I've refactored the call sites I could find easily to use getZero /
If this were smaller I'd check it in for post commit review, but given
that it's is fairly large, it'd be nice to have some extra eyes look
I can bootstrap clang with this change.