Support for addnuw and addnw instruction

addnuw is implemented by verifying that the summation of two operands
is always greater than or equal to both operands.

addnw is for Add No Wrap, which is invoked when both NSW and NUW
flags are enabled.

Note: addnuw can also be implemented by zero extending both the operands,
computing the sum of two and verifying if the N+1th bit is 1, which means
undefined behavior. Not sure which of the two appraoches for addnuw would
be faster? I will submit it as next patch, if required.