It's a pretty crude heuristic, but it stops us from endlessly
rewriting "megamorphic" IC sites.

              pyston interp2.py                         :    6.7s baseline: 6.5 (+3.0%)
              pyston raytrace.py                        :    8.3s baseline: 7.9 (+4.3%)
              pyston nbody.py                           :   10.6s baseline: 10.3 (+3.1%)
              pyston fannkuch.py                        :    7.4s baseline: 7.4 (+0.8%)
              pyston chaos.py                           :   24.2s baseline: 24.6 (-1.5%)
              pyston spectral_norm.py                   :   22.7s baseline: 30.4 (-25.4%)
              pyston fasta.py                           :    9.0s baseline: 8.4 (+7.6%)
              pyston pidigits.py                        :    4.4s baseline: 4.3 (+1.7%)
              pyston richards.py                        :    2.7s baseline: 12.5 (-78.7%)
              pyston deltablue.py                       :    2.7s baseline: 2.6 (+0.9%)
              pyston (geomean-0b9f)                     :    7.6s baseline: 9.0 (-15.2%)

There are a number of regressions; I feel like this is something
we'll be tuning a lot.