Auto merge of #144477 - cjgillot:gvn-index, r=dianqk

GVN: Use a VnIndex in Address projection.

The current implementation of address projections is inconsistent. Indexing semantically relies on the index' value, but the implementation uses the index' place. This PR fixes that by using `ProjectionElem<VnIndex, Ty<'tcx>>` instead of the raw `PlaceElem<'tcx>`.

This is a more principled fix than the workaround in rust-lang/rust#145030.

Author: bors