possible fix for spidermonkey vs v8 differing behavior

bit cast doesn't quite have same behavior btwn spidermonkey and v8
see https://gist.github.com/wahbahdoo/7682630 for a test case
(a == c in v8 but not in spidermonkey with current implementation)

this patch is a possible fix to make spidermonkey behave correctly for bitcast int32x4 to float32x4 back to int32x4
it banks on the fact that even the underlying array is shared, there is no simd api that lets you change the buffer 
however, the last withX test case still wouldn't work (k == a in v8 but not in spidermonkey)

let me know if you have a better idea, thanks!