[libc] Fix bug in UInt comparison operators.

libc/src/__support/CPP/UInt.h

for (size_t i = 0; i < WordCount; ++i) {
       if (val[i] != other.val[i])
         return true;
     }
     return false;
   }
 
   constexpr bool operator>(const UInt<Bits> &other) const {
     for (size_t i = WordCount; i > 0; --i) {
-      if (val[i - 1] <= other.val[i - 1])
+      uint64_t word = val[i - 1];
+      uint64_t other_word = other.val[i - 1];
+      if (word > other_word)
+        return true;
+      else if (word < other_word)
         return false;
     }
-    return true;
+    // Equal
+    return false;
   }
 
   constexpr bool operator>=(const UInt<Bits> &other) const {
     for (size_t i = WordCount; i > 0; --i) {
-      if (val[i - 1] < other.val[i - 1])
+      uint64_t word = val[i - 1];
+      uint64_t other_word = other.val[i - 1];
+      if (word > other_word)
+        return true;
+      else if (word < other_word)
         return false;
     }
+    // Equal
     return true;
   }
 
   constexpr bool operator<(const UInt<Bits> &other) const {
     for (size_t i = WordCount; i > 0; --i) {
-      if (val[i - 1] >= other.val[i - 1])
+      uint64_t word = val[i - 1];
+      uint64_t other_word = other.val[i - 1];
+      if (word > other_word)
         return false;
-    }
+      else if (word < other_word)
         return true;
     }
+    // Equal
+    return false;
+  }
 
   constexpr bool operator<=(const UInt<Bits> &other) const {
     for (size_t i = WordCount; i > 0; --i) {
-      if (val[i - 1] > other.val[i - 1])
+      uint64_t word = val[i - 1];
+      uint64_t other_word = other.val[i - 1];
+      if (word > other_word)
         return false;
+      else if (word < other_word)
+        return true;
     }
+    // Equal
     return true;
   }
 
   // Return the i-th 64-bit word of the number.
   constexpr const uint64_t &operator[](size_t i) const { return val[i]; }
 
   // Return the i-th 64-bit word of the number.
   constexpr uint64_t &operator[](size_t i) { return val[i]; }

libc/test/src/__support/uint128_test.cpp

TEST(LlvmLibcUInt128ClassTest, EqualsTests) {
   ASSERT_TRUE(a1 == a1);
   ASSERT_TRUE(a1 == a2);
   ASSERT_FALSE(a1 == b);
   ASSERT_FALSE(a1 == a_reversed);
   ASSERT_FALSE(a1 == a_lower);
   ASSERT_FALSE(a1 == a_upper);
   ASSERT_TRUE(a_lower != a_upper);
 }
+
+TEST(LlvmLibcUInt128ClassTest, ComparisonTests) {
+  UInt128 a({0xffffffff00000000, 0xffff00000000ffff});
+  UInt128 b({0xff00ff0000ff00ff, 0xf0f0f0f00f0f0f0f});
+  EXPECT_GT(a, b);
+  EXPECT_GE(a, b);
+  EXPECT_LT(b, a);
+  EXPECT_LE(b, a);
+
+  UInt128 x(0xffffffff00000000);
+  UInt128 y(0x00000000ffffffff);
+  EXPECT_GT(x, y);
+  EXPECT_GE(x, y);
+  EXPECT_LT(y, x);
+  EXPECT_LE(y, x);
+
+  EXPECT_LE(a, a);
+  EXPECT_GE(a, a);
+}