[llvm] [Knowbits] Make nuw and nsw support in computeForAddSub optimal (PR #83382)

[Matt Arsenault via llvm-commits]

================
@@ -1903,7 +1903,8 @@ bool AMDGPUDAGToDAGISel::checkFlatScratchSVSSwizzleBug(
   // voffset to (soffset + inst_offset).
   KnownBits VKnown = CurDAG->computeKnownBits(VAddr);
   KnownBits SKnown = KnownBits::computeForAddSub(
-      true, false, CurDAG->computeKnownBits(SAddr),
+      /*Add*/ true, /*NSW*/ false, /*NUW*/ false,
----------------
arsenm wrote:

```suggestion
      /*Add=*/ true, /*NSW*/ false, /*NUW=*/ false,
```

https://github.com/llvm/llvm-project/pull/83382