Fix SpeculateCellOperand ASSERT failure in DFG for 32-bit builds.

Original patch by Filip Pizlo <fpizlo@apple.com> on 2013-08-28
taken from WebKit r154804 (http://trac.webkit.org/changeset/154804).

Task-number: QTBUG-39768
Change-Id: I239f2ee2ec5d1c21f7e50709b8989e744fd87995
Reviewed-by: Zhang Zengbo <zengbo.zhang@gmail.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>