[llvm] [GlobalISel] Add G_ADD for computeNumSignBits (PR #159202)

[Jay Foad via llvm-commits]

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@@ -1959,6 +1959,41 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
 
     break;
   }
+  case TargetOpcode::G_ADD: {
+    Register Src2 = MI.getOperand(2).getReg();
+    unsigned Src2NumSignBits =
+        computeNumSignBits(Src2, DemandedElts, Depth + 1);
+    if (Src2NumSignBits == 1)
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jayfoad wrote:

Possible micro-optimization:
```suggestion
    if (Src2NumSignBits <= 2)
```

https://github.com/llvm/llvm-project/pull/159202