[llvm] [KnownBits] Make `avg{Ceil,Floor}S` and `{s,u}{add,sub}_sat` optimal (PR #110329)

[Yingwei Zheng via llvm-commits]

================
@@ -656,52 +706,8 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
     }
   }
 
-  if (Signed) {
-    if (Add) {
-      if (LHS.isNonNegative() && RHS.isNonNegative()) {
-        // Pos + Pos -> Pos
-        Res.One.clearSignBit();
-        Res.Zero.setSignBit();
-      }
-      if (LHS.isNegative() && RHS.isNegative()) {
-        // Neg + Neg -> Neg
-        Res.One.setSignBit();
-        Res.Zero.clearSignBit();
-      }
-    } else {
-      if (LHS.isNegative() && RHS.isNonNegative()) {
-        // Neg - Pos -> Neg
-        Res.One.setSignBit();
-        Res.Zero.clearSignBit();
-      } else if (LHS.isNonNegative() && RHS.isNegative()) {
-        // Pos - Neg -> Pos
-        Res.One.clearSignBit();
-        Res.Zero.setSignBit();
-      }
-    }
-  } else {
-    // Add: Leading ones of either operand are preserved.
-    // Sub: Leading zeros of LHS and leading ones of RHS are preserved
-    // as leading zeros in the result.
-    unsigned LeadingKnown;
-    if (Add)
-      LeadingKnown =
-          std::max(LHS.countMinLeadingOnes(), RHS.countMinLeadingOnes());
-    else
-      LeadingKnown =
-          std::max(LHS.countMinLeadingZeros(), RHS.countMinLeadingOnes());
-
-    // We select between the operation result and all-ones/zero
-    // respectively, so we can preserve known ones/zeros.
-    APInt Mask = APInt::getHighBitsSet(BitWidth, LeadingKnown);
-    if (Add) {
-      Res.One |= Mask;
-      Res.Zero &= ~Mask;
-    } else {
-      Res.Zero |= Mask;
-      Res.One &= ~Mask;
-    }
-  }
+  KnownBits Res = KnownBits::computeForAddSub(Add, /*NSW=*/Signed,
+                                              /*NUW*/ !Signed, LHS, RHS);
----------------
dtcxzyw wrote:

```suggestion
                                              /*NUW=*/!Signed, LHS, RHS);
```

https://github.com/llvm/llvm-project/pull/110329