[Statepoint] Fix StatepointLoweringInfo::GCTransitionArgs initialization

Summary:
In method SelectionDAGBuilder::LowerStatepoint, array SI.GCTransitionArgs
is initialized from wrong part of ImmutableStatepoint class.
We copy gc args instead of transitions args.

Reviewers: reames, skatkov

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77075