[llvm] [RISCV][GISel] Lower G_SCMP and G_UCMP. (PR #119112)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

We have 
```
/// Should we expand [US]CMP nodes using two selects and two compares, or by
/// doing arithmetic on boolean types
virtual bool shouldExpandCmpUsingSelects(EVT VT) const { return false; }
```
We can add a variant based on LLT.

https://github.com/llvm/llvm-project/pull/119112