I forgot that you need to clear the upper 32 bits for the carry flag to
work properly on ppc64 or else there will be garbage and possibly
incorrect results.
Fixes: https://github.com/llvm/llvm-project/issues/179119
I do not have merge permissions.
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.
