Auto merge of #145737 - cjgillot:gvn-valueset, r=saethlin

GVN: stop hashing opaque values

GVN generates values that are not meant to be unified with any other. For instance `Opaque` (aka we don't know anything), non-deterministic constants and borrows.

The current algorithm generates a unique index, so the generated `Value` will be different from all the existing. This is wasteful, as we should not hash that `Value` at all.

This PR proposes to do this. This involves partially reimplementing a `FxIndexSet`, but yields a small but consistent perf improvement (rust-lang/rust#145737 (comment)).

Author: bors