Ref fields spec on re-assignments can be simplified

[jcouv]

The current spec says:

For a ref reassignment in the form ...
1. x.e1 = ref e2: where x is safe-to-escape at least return only then e2 must have ref-safe-to-escape at least as large as x
2. e1 = ref e2: where e1 is a ref local or ref parameter then e2 must have a safe-to-escape equal to safe-to-escape for e1 and e2 must have ref-safe-to-escape at least as large as ref-safe-to-escape of the ref-safe-to-escape of e1

From offline discussion, we think this can be simplified to:

For a ref reassignment in the form `e1 = ref e2` both of the following must be true:
- e2 must have ref-safe-to-escape at least as large as the ref-safe-to-escape of e1
- e1 must have the same safe-to-escape as e2

The discussion of x.e1 case can be left as a side-note, but is covered by this more general rule.

Once the spec is updated, we can also update the corresponding comment in the compiler implementation.

Relates to test plan #59194 (ref fields)