X64: replace explicit zero-extend with runtime assert

Since the input float64 is in the range of [0, 2^32-1], the upper 32 bits of output register
should be zero.

BUG=
R=bmeurer@chromium.org, titzer@chromium.org

Review URL: https://codereview.chromium.org/644993002

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25027 ce2b1a6d-e550-0410-aec6-3dcde31c8c00