Re-apply "SDAG: Update ChainNodesMatched as nodes are deleted"

My first attempt at this had an overly aggressive assert - chain nodes
will only be removed, but we could hit the assert if a non-chain node
was CSE'd (NodeToMatch, for instance).

This reapplies r271706 by reverting r271713 and fixing an assert.

Original message:

Avoid relying on UB by looking into deleted nodes for a marker value.
Instead, update the list of chain nodes as we go.

llvm-svn: 271733