Avoid the use of large unsigned values in isl unit test

isl_val_int_from_ui takes an 'unsigned long' which has on 32-bit and LLP64
windows systems only 32 bit. Hence, make sure we do not use it with constants
that are larger than 32 bit.

Reported-by: Michael Kruse <llvm@meinersbur.de>
llvm-svn: 279824