Remove prev_index_to_index field from CurrentDepGraph

[Zoxc]

The dep graph currently has 2 ways to map a previous index into a current index. The prev_index_to_index map stores the current index equivalent of a previous index. For indices which are marked green, we also store the same information in the DepNodeColorMap. We actually only need to known the mapping for green nodes however, so this PR removes prev_index_to_index and instead makes use of the DepNodeColorMap.

To avoid racing when promoting a node from the previous session, the encoder lock is now used to ensure only one thread encodes the promoted node. This was previously done by the lock in prev_index_to_index.

This also changes nodes_newly_allocated_in_current_session used to detect duplicate dep nodes to contain both new and previous nodes, which is simpler and can better catch duplicates.

The dep node index encoding used in DepNodeColorMap is tweak to avoid subtraction / addition to optimize accessing the current equivalent of a previous index.

This is based on #138629.

r? @oli-obk

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin