Fix a bug in the x64 elements transition code.

Fix a bug in the x64 elements transition code.

1. The write barrier (RecordWriteStub) expects that pointer stored
points to an initialized object. Specifically, the map must be set
before it is stored.

2. The backing store for smi-only elements can only be reused for
double elements if it is in new-space. Otherwise, we need to allocate
a fresh one because the old one is in pointer-space and the new one
has to be in data-space.

BUG=117037
Committed: https://code.google.com/p/v8/source/detail?r=10968

[Yang, 2012-03-08 14:46:40]

On 2012/03/08 14:41:05, fschneider wrote:

LGTM.