[llvm] [RISCV][GISel] Support G_{U|S}{ADD|SUB}SAT (PR #92935)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Legalize or legalize and select? There are no assembler files.
> 
> See the check line `CHECK,RV32ZBB`. TBH, I think the MIR tests are enough. Should I also add some codegen tests?

End to end codegen tests should generally be preferred 

https://github.com/llvm/llvm-project/pull/92935