cmd/8g: doasm: notfound on y%1 == 0

<pre>Peter hit this on 386.

$ hg id
c240597bbf3e+ tip

$ cat x.go
package main

func main() {
    var y int64
    if y%1 == 0 {
    }
}

$ GOARCH=386 go build x.go
# command-line-arguments
main.main: doasm: notfound t2=0 from=49 to=49 (5)   CMPL    $0,$0
main.main: doasm: notfound t2=0 from=49 to=49 (5)   CMPL    $0,$0

$ GOARCH=amd64 go build x.go
$</pre>
