Rollup merge of #107524 - cjgillot:both-storage, r=RalfJung

Rollup merge of #107524 - cjgillot:both-storage, r=RalfJung

Remove both StorageLive and StorageDead in CopyProp.

Fixes https://github.com/rust-lang/rust/issues/107511

https://github.com/rust-lang/rust/pull/106908 removed StorageDead without the accompanying StorageLive. In loops, execution would see repeated StorageLive, without any StorageDead, which is UB.

So when removing storage statements, we have to remove both StorageLive and StorageDead.

~I also added a MIR validation pass for StorageLive. It may be a bit overzealous.~