[InstCombine] Add transform for ~X + ~Y) -> -2 - Y - X

Proof: https://alive2.llvm.org/ce/z/36FySK

Closes #66787.

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1662,6 +1662,23 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
                                     m_c_UMin(m_Deferred(A), m_Deferred(B))))))
     return BinaryOperator::CreateWithCopiedFlags(Instruction::Add, A, B, &I);
 
+  // (~X) + (~Y) --> -2 - (X + Y)
+  {
+    // To ensure we can save instructions we need to ensure that we consume both
+    // LHS/RHS (i.e they have a `not`).
+    bool ConsumesLHS, ConsumesRHS;
+    if (isFreeToInvert(LHS, LHS->hasOneUse(), ConsumesLHS) && ConsumesLHS &&
+        isFreeToInvert(RHS, RHS->hasOneUse(), ConsumesRHS) && ConsumesRHS) {
+      Value *NotLHS = getFreelyInverted(LHS, LHS->hasOneUse(), &Builder);
+      Value *NotRHS = getFreelyInverted(RHS, RHS->hasOneUse(), &Builder);
+      assert(NotLHS != nullptr && NotRHS != nullptr &&
+             "isFreeToInvert desynced with getFreelyInverted");
+      Value *LHSPlusRHS = Builder.CreateAdd(NotLHS, NotRHS);
+      return BinaryOperator::CreateSub(ConstantInt::get(RHS->getType(), -2),
+                                       LHSPlusRHS);
+    }
+  }
+
   // TODO(jingyue): Consider willNotOverflowSignedAdd and
   // willNotOverflowUnsignedAdd to reduce the number of invocations of
   // computeKnownBits.

llvm/test/Transforms/InstCombine/free-inversion.ll

@@ -117,11 +117,10 @@ define i8 @sub_1(i8 %a, i1 %c, i8 %x, i8 %y) {
 
 define i8 @sub_2(i8 %a, i1 %c, i8 %x, i8 %y) {
 ; CHECK-LABEL: @sub_2(
-; CHECK-NEXT:    [[NX:%.*]] = xor i8 [[X:%.*]], -1
-; CHECK-NEXT:    [[YY:%.*]] = xor i8 [[Y:%.*]], 123
-; CHECK-NEXT:    [[B:%.*]] = select i1 [[C:%.*]], i8 [[NX]], i8 [[YY]]
-; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[A:%.*]], -1
-; CHECK-NEXT:    [[NOT_AB:%.*]] = add i8 [[B]], [[TMP1]]
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[Y:%.*]], -124
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[C:%.*]], i8 [[X:%.*]], i8 [[TMP1]]
+; CHECK-NEXT:    [[TMP3:%.*]] = add i8 [[TMP2]], [[A:%.*]]
+; CHECK-NEXT:    [[NOT_AB:%.*]] = sub i8 -2, [[TMP3]]
 ; CHECK-NEXT:    ret i8 [[NOT_AB]]
 ;
   %nx = xor i8 %x, -1