[Statepoints][NFC] Add Statepoint::getGCResult.

Summary:
Introduce a simple accessor to get the gc_result hanging off of a
statepoint.

(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/D10627

llvm-svn: 241231