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

[via llvm-commits]

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@@ -244,18 +244,30 @@ LIBC_INLINE constexpr bool is_negative(cpp::array<word, N> &array) {
 enum Direction { LEFT, RIGHT };
 
 // A bitwise shift on an array of elements.
+// TODO: Make the result UB when 'offset' is greater or equal to the number of
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lntue wrote:

Or similar to other ones, we can have both safe and unsafe shifts.

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