[AArch64][GlobalISel] Use emitTestBit in selection for G_BRCOND

Partially refactoring, partially fixing a bug.

- We shouldn't use TB(N)ZX unless the bit number is >= 32
- We can fold more than xor using emitTestBit

Also remove a check which isn't relevant anymore + update tests.

Rename select-brcond-of-not.mir to select-brcond-of-binop.mir, since it now
tests more than just G_XOR.

Differential Revision: https://reviews.llvm.org/D88702