[AArch64][GlobalISel] Fold G_ADD into the cset for G_ICMP

When we have a G_ADD which is fed by a G_ICMP on one side, we can fold it into
the cset for the G_ICMP.

e.g. Given

```
%cmp = G_ICMP ... %x, %y
%add = G_ADD %cmp, %z
```

We would normally emit a cmp, cset, and add.

However, `%add` is either `%z` or `%z + 1`. So, we can just use `%z` as the
source of the cset rather than wzr, saving an instruction.

This would probably be cleaner in AArch64PostLegalizerLowering, but we'd need
to change the way we represent G_ICMP to do that, I think. For now, it's
easiest to implement in selection.

This is a 0.1% code size improvement on CTMark/pairlocalalign at -Os.

Example: https://godbolt.org/z/7KdrP8

Differential Revision: https://reviews.llvm.org/D96388

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