Delta-compress register maps in stack maps.

The register maps tend to be similar from stack map to stack map,
so instead of encoding them again, store only the modified ones.

The dex register bitmap stores the delta now - if register has
been modified since the previous stack map, the bit will be set.

The decoding logic scans backwards through stack maps until it
eventfully finds the most recent value of each register.

This CL saves ~2.5% of .oat file size (~10% of stackmap size).

Due to the scan, this makes dex register decoding slower by factor
of 2.5, but that still beats the old algorithm before refactoring.

Test: test-art-host-gtest-stack_map_test
Change-Id: Id5217a329eb757954e0c9447f38b05ec34118f84