src: simplify UnionBytes

Before this commit it was using a tagged union to store the one-byte and
two-byte pointers.

From a `sizeof(UnionBytes)` perspective that makes no difference - there
is a hole between the tag and the union - and it makes the code just a
little harder to reason about, IMO.

PR-URL: #29116
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

src/node_union_bytes.h

@@ -59,47 +59,40 @@ class NonOwningExternalTwoByteResource
 class UnionBytes {
  public:
   UnionBytes(const uint16_t* data, size_t length)
-      : is_one_byte_(false), two_bytes_(data), length_(length) {}
+      : one_bytes_(nullptr), two_bytes_(data), length_(length) {}
   UnionBytes(const uint8_t* data, size_t length)
-      : is_one_byte_(true), one_bytes_(data), length_(length) {}
+      : one_bytes_(data), two_bytes_(nullptr), length_(length) {}
 
   UnionBytes(const UnionBytes&) = default;
   UnionBytes& operator=(const UnionBytes&) = default;
   UnionBytes(UnionBytes&&) = default;
   UnionBytes& operator=(UnionBytes&&) = default;
 
-  bool is_one_byte() const { return is_one_byte_; }
+  bool is_one_byte() const { return one_bytes_ != nullptr; }
   const uint16_t* two_bytes_data() const {
-    CHECK(!is_one_byte_);
     CHECK_NOT_NULL(two_bytes_);
     return two_bytes_;
   }
   const uint8_t* one_bytes_data() const {
-    CHECK(is_one_byte_);
     CHECK_NOT_NULL(one_bytes_);
     return one_bytes_;
   }
   v8::Local<v8::String> ToStringChecked(v8::Isolate* isolate) const {
-    if (is_one_byte_) {
-      CHECK_NOT_NULL(one_bytes_);
+    if (is_one_byte()) {
       NonOwningExternalOneByteResource* source =
-          new NonOwningExternalOneByteResource(one_bytes_, length_);
+          new NonOwningExternalOneByteResource(one_bytes_data(), length_);
       return v8::String::NewExternalOneByte(isolate, source).ToLocalChecked();
     } else {
-      CHECK_NOT_NULL(two_bytes_);
       NonOwningExternalTwoByteResource* source =
-          new NonOwningExternalTwoByteResource(two_bytes_, length_);
+          new NonOwningExternalTwoByteResource(two_bytes_data(), length_);
       return v8::String::NewExternalTwoByte(isolate, source).ToLocalChecked();
     }
   }
   size_t length() { return length_; }
 
  private:
-  bool is_one_byte_;
-  union {
-    const uint8_t* one_bytes_;
-    const uint16_t* two_bytes_;
-  };
+  const uint8_t* one_bytes_;
+  const uint16_t* two_bytes_;
   size_t length_;
 };