This function mixes the bits in the hash value, which improves Dictionary
performance for keys with bad hashes.
PrecommitBenchmark changes with greater than 7% difference:
``````````Dictionary2`,```1456.00`,```1508.00`,```1502.00`,````624.00`,````607.00`,````592.00`,`864.00`,``145.9%
``````````Dictionary3`,```1379.00`,```1439.00`,```1408.00`,````585.00`,````567.00`,````552.00`,`827.00`,``149.8%
````````````Histogram`,````850.00`,````849.00`,````851.00`,```1053.00`,```1049.00`,```1048.00`,`199.00`,``-19.0%
````````````````Prims`,```1999.00`,```2005.00`,```2018.00`,```1734.00`,```1689.00`,```1701.00`,`310.00`,```18.4%
``````````StrSplitter`,```2365.00`,```2334.00`,```2316.00`,```1979.00`,```1997.00`,```2000.00`,`337.00`,```17.0%
```````````````TwoSum`,```1551.00`,```1568.00`,```1556.00`,```1771.00`,```1741.00`,```1716.00`,`165.00`,```-9.6%
Regressions are in benchmarks that use `Int` as dictionary key: we are just
doing more work than previously (hashing an `Int` was an identity function).
rdar://17962402
Swift SVN r21142
