[AArch64][GlobalISel] Fold G_LSHR into test bit calculation

Add support for walking through G_LSHR in `getTestBitReg`. Equivalent to the
code in `getTestBitOperand` in AArch64ISelLowering.

```
(tbz (lshr x, c), b) -> (tbz x, b+c) when b + c is < # bits in x
```

Differential Revision: https://reviews.llvm.org/D74077