X64: Convert smis to holding 32 bits of payload.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index 275dbb5cb21589b58db375485b61fcd552f16180..f4a0598c20368b1ae7e936d43196bd74f8e5f22f 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -36,7 +36,8 @@ namespace internal {
 // ----------------------------------------------------------------------------
 // General helper functions
 
-// Returns true iff x is a power of 2.  Does not work for zero.
+// Returns true iff x is a power of 2 (or zero). Cannot be used with the
+// maximally negative value of the type T (the -1 overflows).
 template <typename T>
 static inline bool IsPowerOf2(T x) {
   return (x & (x - 1)) == 0;