Fix gc on MIPS (live registers were spilled in reversed order).
Add comments on all platforms.

R=koda@google.com

Review URL: https://codereview.chromium.org//785443002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42125 260f80e4-7a28-3924-810f-c04153c831b5