Auto merge of #147493 - cjgillot:single-pin, r=oli-obk

StateTransform: Only load pin field once.

The current implementation starts by transforming all instances of `_1` into `(*_1)`, and then traverses the body again to transform `(*_1)` into `(*(_1.0))`, and again for `Derefer`.

This PR changes the implementation to only traverse the body once. As `_1.0` cannot be not modified inside the body (we just changed its type!), we have no risk of loading from the wrong pointer.

Author: bors