Tell the optimizer the value in unwrap_unchecked

Result::unwrap_unchecked writes to state_ but also assumes it is always
an Ok value. The optimizer fails to remove branches that construct the
Result currently, but succeeds if we tell it that the state_ is an Ok
value before clobbering it.

Repeat the same thing in Result::unwrap_err_unchecked and in
Option::unwrap_unchecked for good measure.

The difference can be seen in here:
https://godbolt.org/z/Gax47shsb

Without it, the signed code generation is terrible. But with the
optimizer hint, it's as good as working with ints directly.

Author: danakj

sus/option/option.h

@@ -847,7 +847,14 @@ class Option final {
   /// verified the value appropriately before use in order to not panic.
   constexpr inline T unwrap_unchecked(
       ::sus::marker::UnsafeFnMarker) && noexcept {
-    return t_.take_and_set_none();
+    if (t_.state() == Some) {
+      return t_.take_and_set_none();
+    } else {
+      // Result::unwrap_unchecked benefits from telling the compiler explicitly
+      // that the other states are never set. Match that here until shown it's
+      // actually not useful.
+      sus::unreachable_unchecked(::sus::marker::unsafe_fn);
+    }
   }
   constexpr inline T unwrap_unchecked(
       ::sus::marker::UnsafeFnMarker) const& noexcept

sus/result/result.h

@@ -857,6 +857,14 @@ class [[nodiscard]] Result final {
   /// Behavior.
   constexpr inline T unwrap_unchecked(
       ::sus::marker::UnsafeFnMarker) && noexcept {
+    if (state_ != ResultState::IsOk) {
+      // This enables Clang to drop any branches that were used to construct
+      // the Result. Without this, the optimizer keeps the whole Result
+      // construction, possibly because the `state_` gets clobbered below?
+      // The signed code version at https://godbolt.org/z/Gax47shsb improves
+      // greatly when the compiler is informed about the UB here.
+      sus::unreachable_unchecked(::sus::marker::unsafe_fn);
+    }
     state_ = ResultState::IsMoved;
     if constexpr (!std::is_void_v<T>) {
       return ::sus::mem::take_and_destruct(::sus::marker::unsafe_fn,
@@ -897,6 +905,11 @@ class [[nodiscard]] Result final {
   /// Behavior.
   constexpr inline E unwrap_err_unchecked(
       ::sus::marker::UnsafeFnMarker) && noexcept {
+    if (state_ != ResultState::IsErr) {
+      // Match the code in unwrap_unchecked, and tell the compiler that the
+      // `state_` is an Err before clobbering it.
+      sus::unreachable_unchecked(::sus::marker::unsafe_fn);
+    }
     state_ = ResultState::IsMoved;
     return ::sus::mem::take_and_destruct(::sus::marker::unsafe_fn,
                                          storage_.err_);