Replace scratch register with XOR swap

I'm getting up to snuff on compiler backend details for a personal project and the `regalloc2` overview and the [follow on article](https://cfallin.org/blog/2022/06/09/cranelift-regalloc2/) have been great.

I'm not going to pretend to completely understand the details here, but the `MachineEnv` requires a scratch register for what I gather is typically swaps, symbolically like this.
```rs
let x = 4;
let y = 5;

let swap = x;
x = y;
y = swap;
```
However there is a way to swap two values without introducing a third variable/register which is via an [XOR swap](https://en.wikipedia.org/wiki/XOR_swap_algorithm):
```rs
x = y ^ x;
y = x ^ y;
x = y ^ x;
```
You shouldn't do this in high level code but should be fine in machine code of course. If an ISA allows integer operations to be applied to float registers then this can be used for both classes.

I'm nearly certain you know of this already, but I wanted to get it out there for the off chance, and if nothing else get details on why it wouldn't work.

首先审查 MachineEnv 的 scratch-register 要求，以及链接的 regalloc2 概览和后续文章。确定 XOR swaps 对相关的寄存器类别和 ISA 是否有效，以及它们是否能保持 allocator 所要求的行为。只有在对是否应以此替换 scratch-register swaps 得出具体且有文档记录的决定后，才算完成。

由索引模型根据 Issue 内容生成。