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

This implements walking over G_ASHR in the same way as `getTestBitOperand` in
AArch64ISelLowering.

```
(tbz (ashr x, c), b) -> (tbz x, b+c) or (tbz x, msb) if b+c is > # bits in x
```

Differential Revision: https://reviews.llvm.org/D73933