Auto merge of #143509 - cjgillot:copy-prop-noborrow, r=tmiasko

Do not unify borrowed locals in CopyProp.

Instead of trying yet another scheme to unify borrowed locals in CopyProp, let's just stop trying. We had already enough miscompilations because of this.

I'm convinced it's possible to have both unification of some borrowed locals and soundness, but I don't have a simple and convincing formulation yet.

Fixes rust-lang/rust#143491

Author: bors