Simplified variable usage

src/build/scope/Variable.go

@@ -6,8 +6,22 @@ import (
 
 // Variable represents a named register.
 type Variable struct {
-	Scope    *Scope
 	Name     string
-	Register cpu.Register
 	Alive    int
+	Depth    uint8
+	Register cpu.Register
+}
+
+// IsAlive returns true if the variable is still alive.
+func (v *Variable) IsAlive() bool {
+	return v.Alive > 0
+}
+
+// Use reduces the lifetime counter by one.
+func (v *Variable) Use() {
+	v.Alive--
+
+	if v.Alive < 0 {
+		panic("incorrect number of variable use calls")
+	}
 }