[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.

v2: Optimize copyExtraInfo() deep copy. For now we assume that only
NodeExtraInfo that have PCSections set require deep copy. Furthermore,
limit the depth of graph search while pre-populating the visited set,
assuming the to-be-replaced subgraph 'From' has limited complexity. An
assertion catches if the maximum depth needs to be increased.

Reviewed By: dvyukov

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