[Statepoints][NFC] Add Statepoint::operator bool()

Summary:
This allows the "if (Statepoint SP = Statepoint(I))" idiom.

(I don't think this change needs review, this was uploaded to
phabricator to provide context for later dependent changes.)

Differential Revision: http://reviews.llvm.org/D10629

llvm-svn: 241232