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