When type-checking binops, LHS of assign-op like `+=` is invariant.
When type-checking binops, LHS of assign-op like `+=` is invariant.

Therefore we cannot coerce it to a supertype the same way that we can
the LHS of `+`.

Addresses issue 52126.

aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust