[IRBuilder] Add gc.statepoint related methods to IRBuilder

Summary:
This adds some more routines to `IRBuilder` around creating calls and
invokes to `gc.statepoint`.  These will be used later.

Reviewers: reames, swaroop.sridhar

Subscribers: sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D13371

llvm-svn: 249596