Implemented a basic optimization

src/build/core/Function.go

@@ -47,6 +47,13 @@ func NewFunction(name string, file *fs.File, body token.List) *Function {
 func (f *Function) Compile() {
 	defer close(f.finished)
 	f.assembler.Label(asm.LABEL, f.Name)
+
+	for _, register := range f.cpu.Input {
+		if f.cpu.IsUsed(register) {
+			f.SaveRegister(register)
+		}
+	}
+
 	f.err = f.CompileTokens(f.Body)
 	f.assembler.Return()
 }