Add "first class" lowering for deopt operand bundles

Summary:
After this change, deopt operand bundles can be lowered directly by
SelectionDAG into STATEPOINT instructions (which are then lowered to a
call or sequence of nop, with an associated __llvm_stackmaps entry0.
This obviates the need to round-trip deoptimization state through
gc.statepoint via RewriteStatepointsForGC.

Reviewers: reames, atrick, majnemer, JosephTremoulet, pgavlin

Subscribers: sanjoy, mcrosier, majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D18257

llvm-svn: 264015