[AArch64][GlobalISel] Fold binops on the true side of G_SELECT

This implements the following folds:

```
G_SELECT cc, (G_SUB 0, %x), %false -> CSNEG %x, %false, inv_cc
G_SELECT cc, (G_XOR x, -1), %false -> CSINV %x, %false, inv_cc
```

This is similar to the folds introduced in
5bc0bd05.

In 5bc0bd05 I mentioned that we may prefer to do
this in AArch64PostLegalizerLowering.

I think that it's probably better to do this in the selector. The way we select
G_SELECT depends on what register banks end up being assigned to it. If we did
this in AArch64PostLegalizerLowering, then we'd end up checking *every* G_SELECT
to see if it's worth swapping operands. Doing it in the selector allows us to
restrict the optimization to only relevant G_SELECTs.

Also fix up some comments in `TryFoldBinOpIntoSelect` which are kind of
confusing IMO.

Example IR: https://godbolt.org/z/3qPGca

Differential Revision: https://reviews.llvm.org/D92860