cmd/compile: SETcc could directly write to memory on 386/amd64

```go
package main

type Foo struct {
	a bool
}

var f Foo
var x int = 2

func main() {
	f.a = x <= 4
}
```

Code generated for main:

```
	main.go:11	0x104bfe4	4883f804		CMPQ $0x4, AX
	main.go:11	0x104bfe8	0f9ec0			SETLE AL
	main.go:11	0x104bfeb	8844240f		MOVB AL, 0xf(SP)
```

should be:

```
	CMPQ $0x4, AX
	SETLE 0xf(SP)
```

more compact, and saves one register.