[InstCombine] Detect (x ^ -x) as a ~Mask

Proof: https://alive2.llvm.org/ce/z/TAFmPw

This is a lemma for clearing up some of the regressions that #84688
causes.

Closes #84868

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -4113,9 +4113,12 @@ static bool isMaskOrZero(const Value *V, bool Not, const SimplifyQuery &Q,
     if (match(V, m_Not(m_Value(X))))
       return isMaskOrZero(X, !Not, Q, Depth);
 
+    // (X ^ -X) is a ~Mask
+    if (Not)
+      return match(V, m_c_Xor(m_Value(X), m_Neg(m_Deferred(X))));
     // (X ^ (X - 1)) is a Mask
-    return !Not &&
-           match(V, m_c_Xor(m_Value(X), m_Add(m_Deferred(X), m_AllOnes())));
+    else
+      return match(V, m_c_Xor(m_Value(X), m_Add(m_Deferred(X), m_AllOnes())));
   case Instruction::Select:
     // c ? Mask0 : Mask1 is a Mask.
     return isMaskOrZero(I->getOperand(1), Not, Q, Depth) &&

llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll

@@ -456,11 +456,10 @@ define i1 @src_is_notmask_shl(i8 %x_in, i8 %y, i1 %cond) {
 define i1 @src_is_notmask_x_xor_neg_x(i8 %x_in, i8 %y, i1 %cond) {
 ; CHECK-LABEL: @src_is_notmask_x_xor_neg_x(
 ; CHECK-NEXT:    [[X:%.*]] = xor i8 [[X_IN:%.*]], 123
-; CHECK-NEXT:    [[NEG_Y:%.*]] = sub i8 0, [[Y:%.*]]
+; CHECK-NEXT:    [[NEG_Y:%.*]] = add i8 [[Y:%.*]], -1
 ; CHECK-NEXT:    [[NOTMASK0:%.*]] = xor i8 [[NEG_Y]], [[Y]]
-; CHECK-NEXT:    [[NOTMASK:%.*]] = select i1 [[COND:%.*]], i8 [[NOTMASK0]], i8 -8
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], [[NOTMASK]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[AND]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[COND:%.*]], i8 [[NOTMASK0]], i8 7
+; CHECK-NEXT:    [[R:%.*]] = icmp ule i8 [[X]], [[TMP3]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %x = xor i8 %x_in, 123