[JSC] Use or64 since it can encode startOfStructureHeap in imm

https://bugs.webkit.org/show_bug.cgi?id=255570
rdar://108171748

Reviewed by Justin Michaud.

StructureAlignedMemoryAllocator ensures that we can compose Structure* with StructureID with BitOr operation.
This patch changes add64 to or64 since or64 can encode bits more nicely in imm field, removing one unnecessary `mov`.

* Source/JavaScriptCore/jit/AssemblyHelpers.cpp:
(JSC::AssemblyHelpers::emitNonNullDecodeZeroExtendedStructureID):

Canonical link: https://commits.webkit.org/263057@main

Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp

@@ -21926,7 +21926,7 @@ IGNORE_CLANG_WARNINGS_END
         LValue maskedStructureID = structureID;
         if constexpr (structureHeapAddressSize < 4 * GB)
             maskedStructureID = m_out.bitAnd(structureID, m_out.constInt32(StructureID::structureIDMask));
-        return m_out.add(m_out.constIntPtr(g_jscConfig.startOfStructureHeap), m_out.zeroExtPtr(maskedStructureID));
+        return m_out.bitOr(m_out.constIntPtr(g_jscConfig.startOfStructureHeap), m_out.zeroExtPtr(maskedStructureID));
 #endif
     }
 

Source/JavaScriptCore/jit/AssemblyHelpers.cpp

@@ -442,11 +442,9 @@ void AssemblyHelpers::emitNonNullDecodeZeroExtendedStructureID(RegisterID source
     if constexpr (structureHeapAddressSize >= 4 * GB) {
         ASSERT(structureHeapAddressSize == 4 * GB);
         move(source, dest);
-        add64(TrustedImm64(g_jscConfig.startOfStructureHeap), dest);
-    } else {
+    } else
         and32(TrustedImm32(StructureID::structureIDMask), source, dest);
-        add64(TrustedImm64(g_jscConfig.startOfStructureHeap), dest);
-    }
+    or64(TrustedImm64(g_jscConfig.startOfStructureHeap), dest);
 #else // not CPU(ADDRESS64)
     move(source, dest);
 #endif