In order to be able to efficiently iterate backwards over instructions
and removing or replacing instructions in the graph we want them to
be a double-linked list inside basic blocks. The list has the following
1. Block entry instructions do not have a previous instruction.
2. The last instruction in a block may or may not have a next instruction:
- Branches have a NULL-successor.
- Normal block exits have a block entry instructions as successor.
This CL also makes the accessor for previous and next instruction in this
list non-virtual. This avoidis the current code duplication there.
Review URL: https://chromiumcodereview.appspot.com//10665022
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@9297 260f80e4-7a28-3924-810f-c04153c831b5
vegorov@google.com