cmd/compile: "bad code" with very large return value

```go
package p

type S struct {
	i interface {
		f() [107374100]int
	}
}
```

Compiling this with 1.9beta1 results in:

```
$ go tool compile x.go
compile: bad code
```

Same for 1.8.

Unlike its brethren, #20779 and #20778, the size of the return value here seems reasonable enough that it might be worth briefly investigating for 1.10, just in case.

Found by go-fuzz.
