cmd/compile: miscompilation of multi-assignment to named result parameters in function with defer+recover

This program used to run correctly with Go 1.15, but fails with Go 1.16 beta 1:

```
package main

func main() {
	if f() {
		panic("FAIL")
	}
}

func f() (bad bool) {
	defer func() {
		recover()
	}()
	var p *int
	bad, _ = true, *p
	return
}
```

/cc @cuonglm 