Fix bmalloc's PackedAlignedPtr for X86_64 canonical addresses

See https://bugs.webkit.org/show_bug.cgi?id=214142

Suitable for upstream.


diff --git a/Source/bmalloc/bmalloc/Packed.h b/Source/bmalloc/bmalloc/Packed.h
index a51809445086..a0d1e06a9508 100644
--- webkit/Source/bmalloc/bmalloc/Packed.h.~1~	2024-11-24 18:41:16.000000000 +0000
+++ webkit/Source/bmalloc/bmalloc/Packed.h	2024-11-24 19:03:37.356568453 +0000
@@ -37,6 +37,8 @@
 
 namespace bmalloc {
 
+template<typename T> constexpr unsigned countOfBits = sizeof(T) * CHAR_BIT;
+
 template<typename T>
 class Packed {
 public:
@@ -145,6 +147,12 @@
 #endif
         if (isAlignmentShiftProfitable)
             value <<= alignmentShiftSize;
+
+#if BCPU(X86_64) && !(BOS(DARWIN) || BOS(LINUX) || BOS(WINDOWS))
+        constexpr unsigned shiftBits = countOfBits<uintptr_t> - BOS_EFFECTIVE_ADDRESS_WIDTH;
+        value = (std::bit_cast<intptr_t>(value) << shiftBits) >> shiftBits;
+#endif
+
         return std::bit_cast<T*>(value);
     }
 
@@ -158,6 +166,7 @@
 #else
         memcpy(m_storage.data(), std::bit_cast<uint8_t*>(&value) + (sizeof(void*) - storageSize), storageSize);
 #endif
+        BASSERT(get() == passedValue);
     }
 
     void clear()