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

This implements the following optimization:

```
(tbz (shl x, c), b) -> (tbz x, b-c)
```

Which appears in `getTestBitOperand` in AArch64ISelLowering.cpp.

If we test bit `b` of `shl x, c`, we can fold away the `shl` by looking `c` bits
to the right of `b` in `x` when this fits in the type. So, we can just test the
`b-c`th bit.

Differential Revision: https://reviews.llvm.org/D73924