[llvm] [DAG] Fold (umin (sub a b) a) -> (usubo a b); (select usubo.1 a usubo.0) (PR #161651)

[Chaitanya Koparkar via llvm-commits]

ckoparkar wrote:

To fix the test failure I need to update the patterns in `CodeGen/AMDGPU/llvm.set.rounding.ll` since the fold also applies to it. The DAG for `set_rounding`:

```
Legalized selection DAG: %bb.0 's_set_rounding:'
SelectionDAG has 17 nodes:
  t0: ch,glue = EntryToken
  t2: i32,ch = CopyFromReg t0, Register:i32 %0
                t8: i32 = sub t2, Constant:i32<4>
              t9: i32 = umin t2, t8
            t11: i32 = shl t9, Constant:i32<2>
          t12: i64 = srl Constant:i64<-5242644231586798321>, t11
        t13: i32 = truncate t12
      t15: i32 = llvm.amdgcn.readfirstlane TargetConstant:i32<3375>, t13
    t18: ch = llvm.amdgcn.s.setreg t0, TargetConstant:i32<3416>, TargetConstant:i32<6145>, t15
  t5: ch = RET_GLUE t18
```

https://github.com/llvm/llvm-project/pull/161651