[SelectionDAG] Expand [US]CMP using arithmetic on boolean values instead of selects (#98774)

The previous expansion of [US]CMP was done using two selects and two
compares. It produced decent code,

[SelectionDAG] Expand [US]CMP using arithmetic on boolean values instead of selects (#98774)

The previous expansion of [US]CMP was done using two selects and two
compares. It produced decent code, but on many platforms it is better to
implement [US]CMP nodes by performing the following operation:

```
[us]cmp(x, y) = (x [us]> y) - (x [us]< y)
```

This patch adds this new expansion, as well as a hook in TargetLowering to allow some targets to still use the select-based approach. AArch64 and SystemZ are currently the only targets to prefer the former approach, but other targets may also start to use it if it provides for better codegen.

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