A few adjustments in big integer shr_assign

[tcoratger]

• If the number of bits shifted is larger than N * 64, the function can be exited at the beginning without having to do the rest of the calculations.
• As the limbs are traversed from the end to the beginning, it is generally less expensive to do a while starting from the end to avoid looking for an index at the opposite end of the table at each iteration.