func markReachable(b *BasicBlock) {
	b.Index = -1
	for _, succ := range b.Succs {
		if succ.Index == 0 {
			markReachable(succ)
		}
	}
}
markReachable sets Index=-1 for all blocks reachable from b.
package ssa
import (
	"fmt"
	"os"
)
const debugBlockOpt = false