make gc branch compile on Win32.

src/compiler-intrinsics.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 32 matching lines @@
 };
 
 #ifdef __GNUC__
 int CompilerIntrinsics::CountTrailingZeros(uint32_t value) {
   return __builtin_ctz(value);
 }
 
 int CompilerIntrinsics::CountLeadingZeros(uint32_t value) {
   return __builtin_clz(value);
 }
+
+#elif defined(_MSC_VER)
+
+#pragma intrinsic(_BitScanForward)
+#pragma intrinsic(_BitScanReverse)
+
+int CompilerIntrinsics::CountTrailingZeros(uint32_t value) {
+  unsigned long result;
+  _BitScanForward(&result, static_cast<long>(value));
+  return static_cast<int>(result);
+}
+
+int CompilerIntrinsics::CountLeadingZeros(uint32_t value) {
+  unsigned long result;
+  _BitScanReverse(&result, static_cast<long>(value));
+  return 31 - static_cast<int>(result);
+}
+
 #else
 #error Unsupported compiler
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_COMPILER_INTRINSICS_H_