Fix UInt128 eq_mask to always return 2^128-1 or 0
Fix UInt128 eq_mask to always return 2^128-1 or 0

The masks from both halves of the int should be combined so that eq_mask
always returns all 1s if the numbers are equal or all 0s otherwise.

FStarLang/kremlin,FStarLang/kremlin,FStarLang/kremlin,FStarLang/kremlin