ValueMapper: Separate mapping of distinct and uniqued nodes (again)

Since the result of a mapped distinct node is known up front, it's more
efficient to map them separately from uniqued nodes.  This commit pulls
them out of the post-order traversal and stores them in a worklist to be
remapped at the top-level.

This is essentially reapplying r244181 ("ValueMapper: Rotate distinct
node remapping algorithm") to the new iterative algorithm from r265456
("ValueMapper: Rewrite Mapper::mapMetadata without recursion").

Now that the traversal logic only handles uniqued MDNodes, it's much
simpler to inline it all into MDNodeMapper::createPOT (I've killed the
MDNodeMapper::push and MDNodeMapper::tryToPop helpers and localized the
traversal worklist).

The resulting high-level algorithm for MDNodeMapper::map now looks like
this:

  - Distinct nodes are immediately mapped and added to
    MDNodeMapper::DistinctWorklist using MDNodeMapper::mapDistinctNode.

  - Uniqued nodes are mapped via MDNodeMapper::mapTopLevelUniquedNode,
    which traverses the transitive uniqued subgraph of a node to
    calculate uniqued node mappings in bulk.

      - This is a simplified version of MDNodeMapper::map from before
        this commit (originally r265456) that doesn't traverse through
        any distinct nodes.

      - Distinct nodes are added to MDNodeMapper::DistinctWorklist via
        MDNodeMapper::mapDistinctNode.

      - This uses MDNodeMapper::createPOT to fill a
        MDNodeMapper::UniquedGraph (a post-order traversal and side
        table), UniquedGraph::propagateChanges to track which uniqued
        nodes need to change, and MDNodeMapper::mapNodesInPOT to create
        the uniqued nodes.

      - Placeholders for forward references are now only needed when
        there's a uniquing cycle (a cycle of uniqued nodes unbroken by
        distinct nodes).  This is the key functionality change that
        we're reintroducing (from r244181).  As of r265456, a temporary
        forward reference might be needed for any cycle that involved
        uniqued nodes.

  - After mapping the first node appropriately, MDNodeMapper::map works
    through MDNodeMapper::DistinctWorklist.  For each distinct node, its
    operands are remapped with MDNodeMapper::mapDistinctNode and
    MDNodeMapper::mapTopLevelUniquedNode until all nodes have been
    mapped.

Sadly there's nothing observable I can test here; no real functionality
change, just a compile-time speedup from reduced malloc traffic.

llvm-svn: 266537