This fixes golang/go#44830 for GopherJS.

In the following example type T is *types.Named, not *types.Pointer, but
the underlying type is:

```go
type T *struct{ y int }
var q T = &struct{ y int }{}
q.y = 7
```

The panic was detected by gorepo test `fixedbugs/issue23017.go`.

GopherJS breaks lhs expression evaluation order defined by the spec.
Unfortunately, it's not easy to fix and would likely generate a lot more
code for multi-assignments.

An efficient fix would require writing some sort of analysis to
determine if assignments are likely to influence each other, and at the
moment I can't think of how such analysis would work. Without it we
would have to create a whole bunch of extra temporary variables for any
multi-assignment, which will likely lead to a significant increase in
the artifact size. Considering nobody reported this issue so far, I'm
inclined to punt on this issue for now.

gopherjs#1063 tracks this bug.