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.

Start by reviewing the MachineEnv scratch-register requirement and the linked regalloc2 overview and follow-on article. Determine whether XOR swaps are valid for the relevant register classes and ISAs, and whether they preserve the allocator's required behavior. Done means reaching a concrete, documented decision about whether this should replace scratch-register swaps.

Written by the indexing model from the issue text.