[SelectionDAG] Transitively copy NodeExtraInfo on RAUW

During legalization of the SelectionDAG, some nodes are replaced with
arch-specific nodes. These may be complex nodes, where the root node no
longer corresponds to the node that should carry the extra info.

Fix the issue by copying extra info to the new node and all its new
transitive operands during RAUW. See code comments for more details.

This fixes the remaining pcsections-atomics.ll tests on X86.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D144677