nir: Handle large unsigned values in opt_algebraic.

nir: Handle large unsigned values in opt_algebraic.

The next patch adds an algebraic rule that uses the constant 0xff00ff00.

Without this change, the build fails with

   return hex(struct.unpack('I', struct.pack('i', self.value))[0])
   struct.error: 'i' format requires -2147483648 <= number <= 2147483647

The hex() function handles integers of any size, and assigning a
negative value to an unsigned does what we want in C. The pack/unpack is
unnecessary (and as we see, buggy).

Reviewed-by: Dylan Baker <baker.dylan.c@gmail.com>