[ORC] Simplify intra-graph dependence tracking in ObjectLinkingLayer.

ObjectLinkingLayer::registerDependencies used to propagate external symbol
dependencies (dependencies on symbols outside the current graph) to all nodes.
Since ebe8733, which merged addDependencies into notifyEmitted, the
notifyEmitted function will propagate intra-graph dependencies, so
registerDependencies no longer needs to do this.

This patch updates ObjectLinkingLayer::registerDependencies to just propagate
named dependencies (on both internal and external symbols) through anonymous
blocks, leaving the rest of the work to ExecutionSession::notifyEmitted.
It also choses a key symbol to use for blocks containing multiple symbols. The
result is both easier to read and faster.

Author: lhames