Optimize stack map decoding.

We usually read several consecutive varints.
Add helper method optimized for that use case
(ideally reading 8 varints from single load).

This improves app startup by 0.4% (maps,speed).
PMD on golem seems to get around 5% faster.
CodeInfo::Decode on its own is 25% faster.

Bug: 133257467
Test: ./art/test.py -b --host --64
Change-Id: Iaf7e8469ed6397b1d1d4102e409b5731f7229557
5 files changed