Optimize IntObjectHashMap handling of negative keys.

Motivation:

The hashIndex method currently uses a conditional to handle negative
keys. This could be done without a conditional to slightly improve
performance.

Modifications:

Modified hashIndex() to avoid using a conditional.

Result:

Slight performance improvement to hashIndex().
@nmittler - Definitely not the biggest performance hit but it is an extra load and store operation (may shuffle stuff out of registers).  Not a big deal but just wanted to get your thoughts on it.