go/types, types2: panic: infinite recursion in unification with go1.21.0 [1.21 backport]

@findleyr requested issue #61879 to be considered for backport to the next 1.21 minor release.

> @griesemer and I will try to fix this for Go1.21.1.
> 
> @gopherbot please backport this to 1.21. It is a new panic during type checking.
