[PATCH] ValueMapper: Rewrite Mapper::mapMetadata without recursion

[Duncan P. N. Exon Smith via llvm-commits]

This commit completely rewrites Mapper::mapMetadata (the implementation
of llvm::MapMetadata) using an iterative algorithm.  The guts of the new
algorithm are in MDNodeMapper::map, the entry function in a new class.

Previously, Mapper::mapMetadata performed a recursive exploration of the
graph with eager "just in case there's a reason" malloc traffic.

The new algorithm has these benefits:

  - New nodes and temporaries are not created eagerly.
  - Uniquing cycles are not duplicated (see new unit test).
  - No recursion.

Given a node to map, it does this:

 1. Use a worklist to perform a post-order traversal of the transitively
    referenced unmapped nodes.

 2. Track which nodes will change operands, and which will have new
    addresses in the mapped scheme.  Propagate the changes through the
    POT until fixed point, to pick up uniquing cycles that need to
    change.

 3. Map all the distinct nodes without touching their operands.  If
    RF_MoveDistinctMetadata, they get mapped to themselves; otherwise,
    they get mapped to clones.

 4. Map the uniqued nodes (bottom-up), lazily creating temporaries for
    forward references as needed.

 5. Remap the operands of the distinct nodes.

MapMetadata barely shows up in the profiles of llvm-link I've looked at
with the new algorithm.

It would be nice to break the strange remaining recursion on the Value
side: MapValue => materializeInitFor => RemapInstruction => MapValue.  I
think we could do this by somehow leaking (at least an opaque version
of) the "Mapper" context into the API, perhaps by sending a callback
function to materializeInitFor.


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