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().
I added it for readability of the expression below.  Not sure exactly what the compiler would do here ... do you think it matters all that much WRT performance?