Refactored code structure

src/build/cpu/CPU.go

@@ -30,3 +30,14 @@ func (c *CPU) FindFree(registers []Register) (Register, bool) {
 
 	return 0, false
 }
+
+func (c *CPU) MustUseFree(registers []Register) Register {
+	register, exists := c.FindFree(registers)
+
+	if !exists {
+		panic("no free registers")
+	}
+
+	c.Use(register)
+	return register
+}