[AArch64][GlobalISel] Fold G_XOR into TB(N)Z bit calculation

This ports the existing case for G_XOR from `getTestBitOperand` in
AArch64ISelLowering into GlobalISel.

The idea is to flip between TBZ and TBNZ while walking through G_XORs.

Let's say we have

```
tbz (xor x, c), b
```

Let's say the `b`-th bit in `c` is 1. Then

- If the `b`-th bit in `x` is 1, the `b`-th bit in `(xor x, c)` is 0.
- If the `b`-th bit in `x` is 0, then the `b`-th bit in `(xor x, c)` is 1.

So, then

```
tbz (xor x, c), b == tbnz x, b
```

Let's say the `b`-th bit in `c` is 0. Then

- If the `b`-th bit in `x` is 1, the `b`-th bit in `(xor x, c)` is 1.
- If the `b`-th bit in `x` is 0, then the `b`-th bit in `(xor x, c)` is 0.

So, then

```
tbz (xor x, c), b == tbz x, b
```

Differential Revision: https://reviews.llvm.org/D73929