[AArch64][GlobalISel] Select G_SELECT cc, t, (G_SUB 0, x) -> CSNEG t, x, cc

When we see

```
%sub = G_SUB 0, %x
%select = G_SELECT %cc, %t, %sub
```

Fold away the G_SUB by producing

```
%select = CSNEG %t, %x, cc
```

Simple IR example: https://godbolt.org/z/K8TEnh

This is valid on both sides of the select, but for now, just handle one side.
It may make more sense to handle swapping sides during post-legalizer lowering.

Differential Revision: https://reviews.llvm.org/D90723