Add a "gc-transition" operand bundle

Summary:
This adds a new kind of operand bundle to LLVM denoted by the
`"gc-transition"` tag.  Inputs to `"gc-transition"` operand bundle are
lowered into the "transition args" section of `gc.statepoint` by
`RewriteStatepointsForGC`.

This removes the last bit of functionality that was unsupported in the
deopt bundle based code path in `RewriteStatepointsForGC`.

Reviewers: pgavlin, JosephTremoulet, reames

Subscribers: sanjoy, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16342

llvm-svn: 258338