Enable dex diff check on debug builds.

Dex diff checks that the input and output dex files are equivalent at
the IR level. Normally enabled by passing -v to dexlayout, dex diff is
now enabled for debug builds as well.

Bug: 36107940
Test: mm test-art-host
Change-Id: I8c5ebb6a7cc0f699299a7200e8bc019762601feb