Avoid an unnecessary ComputeModifiedUtf8Hash().

The DescriptorHashPair already has the hash as the member
`second`, so avoid recalculating it in release mode.

Also split "HashEquals" classes into separate "Hash" and
"Equals" classes to disambiguate their operators in traces.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 175869411
Change-Id: I87cbe613778c7d310ba5c2bf437729244780472c
4 files changed