disable rewriting

Right now, we spend *a lot* of time trying to do rewrites that are doomed to fail. We need to detect such cases and stop trying to rewrite them.

Trying the simplest possible thing here: if the rewrite for a particular instruction fails once, don't try to rewrite it again. We might want to tweak this, but it's certainly an improvement over what we do now.

I'll let the benchmark speak for itself:

```
# master
$ time ./pyston_release -qb ../test/tests/keyword_ics.py > ~/out

real	1m48.814s
user	1m43.098s
sys	0m5.412s

# this commit
$ time ./pyston_release -qb ../test/tests/keyword_ics.py > ~/out

real	0m41.883s
user	0m37.130s
sys	0m4.648s
````

(I chose `keyword_ics.py` because it has `# expected: statfail`. It bails on function calls with keyword arguments.)