Add -e to test generation script

The -e flag exits the script with a non-zero code if any subcommand
fails. This flag allows us to notice as early as possible if the
test was not properly regenerated using a command like:

$ create_ll.sh t.c && opt < t.ll -polly ...

The above pattern is useful when iteratively developing a test case
to guard against un-noticed syntax errors.

Differential Revision: http://reviews.llvm.org/D5276