A register allocation bugfix.

A register allocation bugfix.

The function that prepares a virtual frame slot for writing (in order
to preserve the copy-on-write semantics of aliased frame elements) can
allocate registers, which may spill one from the frame.  If we're
unlucky, the spilled register can be the source register for the frame
element write.  In that case, ensure we do the write from memory.

Review URL: http://codereview.chromium.org/115125

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1904 ce2b1a6d-e550-0410-aec6-3dcde31c8c00