Avoid shifting U64 by 64
Avoid shifting U64 by 64

Triggers undefined behavior in C (ironically uncaught by F*'s
verification). See https://github.com/FStarLang/kremlin/issues/48.

FStarLang/FStar,FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,A-Manning/FStar