[CodeGen][SDAG] Remove CombinedNodes SmallPtrSet (#94609)

This "small" set grows quite large and it's more performant to store
whether a node has been combined before in the node itself.

As this information is only relevant for nodes that are currently not in
the worklist, add a second state to the CombinerWorklistIndex (-2) to
indicate that a node is currently not in a worklist, but was combined
before.

This brings a substantial performance improvement.