[libc] [llvm] [libc] Refactor `BigInt` (PR #86137)

[Nick Desaulniers via llvm-commits]

================
@@ -930,9 +870,67 @@ struct BigInt {
   // Return the i-th word of the number.
   LIBC_INLINE constexpr WordType &operator[](size_t i) { return val[i]; }
 
-  LIBC_INLINE WordType *data() { return val; }
+private:
+  LIBC_INLINE friend constexpr int cmp(const BigInt &lhs, const BigInt &rhs) {
+    const auto compare = [](WordType a, WordType b) {
+      return a == b ? 0 : a > b ? 1 : -1;
+    };
+    if constexpr (Signed) {
+      const bool lhs_is_neg = lhs.is_neg();
+      const bool rhs_is_neg = rhs.is_neg();
+      if (lhs_is_neg != rhs_is_neg)
+        return rhs_is_neg ? 1 : -1;
+    }
+    for (size_t i = WORD_COUNT; i-- > 0;)
+      if (auto cmp = compare(lhs[i], rhs[i]); cmp != 0)
+        return cmp;
+    return 0;
+  }
+
+  LIBC_INLINE constexpr void bitwise_not() {
+    for (auto &part : val)
+      part = ~part;
+  }
+
+  LIBC_INLINE constexpr void negate() {
+    bitwise_not();
+    increment();
+  }
+
+  LIBC_INLINE constexpr void increment() {
+    multiword::add_with_carry(val, cpp::array<WordType, 1>{1});
+  }
+
+  LIBC_INLINE constexpr void decrement() {
+    multiword::add_with_carry(val, cpp::array<WordType, 1>{1});
+  }
 
-  LIBC_INLINE const WordType *data() const { return val; }
+  LIBC_INLINE constexpr void extend(size_t index, bool is_neg) {
+    const WordType value = is_neg ? cpp::numeric_limits<WordType>::max()
+                                  : cpp::numeric_limits<WordType>::min();
+    for (size_t i = index; i < WORD_COUNT; ++i)
+      val[i] = value;
----------------
nickdesaulniers wrote:

```suggestion
    for (auto &part : val)
      part = value;
```

https://github.com/llvm/llvm-project/pull/86137