cmd/compile: error message for invalid type switch cases less helpful in Go1.18

```Go
	var i I
	var s string

	switch i {
	case s:
	}
```
produces
```
invalid operation: cannot compare s == i (mismatched types string and I)
```
The compiler at 1.17 reported
```
invalid case s in switch on i (mismatched types string and I)
```
