Provide a tagged allocation top pointer.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index d23b2e3c3bfaaa7ca91c9ea1c19d4eac6f3b37a9..4a1269dbd946bc1547750c58a485bf587e7abe28 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -304,6 +304,7 @@ const intptr_t kPointerAlignmentMask = kPointerAlignment - 1;
 // Desired alignment for double values.
 const intptr_t kDoubleAlignment = 8;
 const intptr_t kDoubleAlignmentMask = kDoubleAlignment - 1;
+const intptr_t kDoubleAlignmentMaskTagged = kDoubleAlignmentMask - 1;
 
 // Desired alignment for 128 bit SIMD values.
 const intptr_t kSimd128Alignment = 16;